Re: [Xen-devel] [PATCH v5 1/7] xen/arm: Introduce alternative runtime patchingo

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

CC'ing Konrad.

Konrad,

I'll defer to you on this patch.

On Wed, 20 Jul 2016, Julien Grall wrote:
> Some of the processor erratum will require to modify code sequence.
> As those modifications may impact the performance, they should only
> be enabled on affected cores. Furthermore, Xen may also want to take
> advantage of new hardware features coming up with v8.1 and v8.2.
> 
> This patch adds an infrastructure to patch Xen during boot time
> depending on the "features" available on the platform.
> 
> This code is based on the file arch/arm64/kernel/alternative.c in
> Linux 4.6-rc3. Any references to arm64 have been dropped to make the
> code as generic as possible.
> 
> When Xen is creating the page tables, all the executable sections
> (.text and .init.text) will be marked read-only and then enforced by
> setting SCTLR.WNX.
> 
> Whilst it might be possible to mark those entries read-only after
> Xen has been patched, we would need extra care to avoid possible
> TLBs conflicts (see D4-1732 in ARM DDI 0487A.i) as all
> physical CPUs will be running.
> 
> All the physical CPUs have to be brought up before patching Xen because
> each cores may have different errata/features which require code
> patching. The only way to find them is to probe system registers on
> each CPU.
> 
> To avoid extra complexity, it is possible to create a temporary
> writeable mapping with vmap. This mapping will be used to write the
> new instructions.
> 
> Lastly, runtime patching is currently not necessary for ARM32. So the
> code is only enabled for ARM64.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> 
> ---
>     Changes in v5:
>         - Rebase on the latest staging
> 
>     Changes in v4:
>         - Fix build when ALTERNATIVE is not enabled
>         - Fix typo
>         - Move .altinstructions in init.data
> 
>     Changes in v3:
>         - .altinstr_replacement should live in init.text
>         - Add a comment to explain when apply_alternatives_all should be
>         called.
> 
>     Changes in v2:
>         - Use hard tabs in Kconfig
>         - Update the copyright year
>         - Update the commit message to explain the extra mapping
> ---
>  xen/arch/arm/Kconfig              |   5 +
>  xen/arch/arm/Makefile             |   1 +
>  xen/arch/arm/alternative.c        | 221 
> ++++++++++++++++++++++++++++++++++++++
>  xen/arch/arm/setup.c              |   7 ++
>  xen/arch/arm/xen.lds.S            |   9 ++
>  xen/include/asm-arm/alternative.h | 165 ++++++++++++++++++++++++++++
>  xen/include/asm-arm/arm64/insn.h  |  16 +++
>  7 files changed, 424 insertions(+)
>  create mode 100644 xen/arch/arm/alternative.c
>  create mode 100644 xen/include/asm-arm/alternative.h
> 
> diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig
> index 6231cd5..0141bd9 100644
> --- a/xen/arch/arm/Kconfig
> +++ b/xen/arch/arm/Kconfig
> @@ -14,6 +14,7 @@ config ARM_64
>       def_bool y
>       depends on 64BIT
>       select HAS_GICV3
> +     select ALTERNATIVE
>  
>  config ARM
>       def_bool y
> @@ -46,6 +47,10 @@ config ACPI
>  config HAS_GICV3
>       bool
>  
> +# Select ALTERNATIVE if the architecture supports runtime patching
> +config ALTERNATIVE
> +     bool
> +
>  endmenu
>  
>  source "common/Kconfig"
> diff --git a/xen/arch/arm/Makefile b/xen/arch/arm/Makefile
> index b264ed4..74bd7b8 100644
> --- a/xen/arch/arm/Makefile
> +++ b/xen/arch/arm/Makefile
> @@ -4,6 +4,7 @@ subdir-y += platforms
>  subdir-$(CONFIG_ARM_64) += efi
>  subdir-$(CONFIG_ACPI) += acpi
>  
> +obj-$(CONFIG_ALTERNATIVE) += alternative.o
>  obj-y += bootfdt.o
>  obj-y += cpu.o
>  obj-y += cpufeature.o
> diff --git a/xen/arch/arm/alternative.c b/xen/arch/arm/alternative.c
> new file mode 100644
> index 0000000..d00f98e
> --- /dev/null
> +++ b/xen/arch/arm/alternative.c
> @@ -0,0 +1,221 @@
> +/*
> + * alternative runtime patching
> + * inspired by the x86 version
> + *
> + * Copyright (C) 2014-2016 ARM Ltd.
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2 as
> + * published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + * GNU General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public License
> + * along with this program.  If not, see <http://www.gnu.org/licenses/>.
> + */
> +
> +#include <xen/config.h>
> +#include <xen/init.h>
> +#include <xen/types.h>
> +#include <xen/kernel.h>
> +#include <xen/mm.h>
> +#include <xen/vmap.h>
> +#include <xen/smp.h>
> +#include <xen/stop_machine.h>
> +#include <asm/alternative.h>
> +#include <asm/atomic.h>
> +#include <asm/byteorder.h>
> +#include <asm/cpufeature.h>
> +#include <asm/insn.h>
> +#include <asm/page.h>
> +
> +#define __ALT_PTR(a,f)      (u32 *)((void *)&(a)->f + (a)->f)
> +#define ALT_ORIG_PTR(a)     __ALT_PTR(a, orig_offset)
> +#define ALT_REPL_PTR(a)     __ALT_PTR(a, alt_offset)
> +
> +extern const struct alt_instr __alt_instructions[], __alt_instructions_end[];
> +
> +struct alt_region {
> +    const struct alt_instr *begin;
> +    const struct alt_instr *end;
> +};
> +
> +/*
> + * Check if the target PC is within an alternative block.
> + */
> +static bool_t branch_insn_requires_update(const struct alt_instr *alt,
> +                                          unsigned long pc)
> +{
> +    unsigned long replptr;
> +
> +    if ( is_active_kernel_text(pc) )
> +        return 1;
> +
> +    replptr = (unsigned long)ALT_REPL_PTR(alt);
> +    if ( pc >= replptr && pc <= (replptr + alt->alt_len) )
> +        return 0;
> +
> +    /*
> +     * Branching into *another* alternate sequence is doomed, and
> +     * we're not even trying to fix it up.
> +     */
> +    BUG();
> +}
> +
> +static u32 get_alt_insn(const struct alt_instr *alt,
> +                        const u32 *insnptr, const u32 *altinsnptr)
> +{
> +    u32 insn;
> +
> +    insn = le32_to_cpu(*altinsnptr);
> +
> +    if ( insn_is_branch_imm(insn) )
> +    {
> +        s32 offset = insn_get_branch_offset(insn);
> +        unsigned long target;
> +
> +        target = (unsigned long)altinsnptr + offset;
> +
> +        /*
> +         * If we're branching inside the alternate sequence,
> +         * do not rewrite the instruction, as it is already
> +         * correct. Otherwise, generate the new instruction.
> +         */
> +        if ( branch_insn_requires_update(alt, target) )
> +        {
> +            offset = target - (unsigned long)insnptr;
> +            insn = insn_set_branch_offset(insn, offset);
> +        }
> +    }
> +
> +    return insn;
> +}
> +
> +static int __apply_alternatives(const struct alt_region *region)
> +{
> +    const struct alt_instr *alt;
> +    const u32 *origptr, *replptr;
> +    u32 *writeptr, *writemap;
> +    mfn_t text_mfn = _mfn(virt_to_mfn(_stext));
> +    unsigned int text_order = get_order_from_bytes(_end - _start);
> +
> +    printk("Patching kernel code\n");
> +
> +    /*
> +     * The text section is read-only. So re-map Xen to be able to patch
> +     * the code.
> +     */
> +    writemap = __vmap(&text_mfn, 1 << text_order, 1, 1, PAGE_HYPERVISOR,
> +                      VMAP_DEFAULT);
> +    if ( !writemap )
> +    {
> +        printk("alternatives: Unable to map the text section\n");
> +        return -ENOMEM;
> +    }
> +
> +    for ( alt = region->begin; alt < region->end; alt++ )
> +    {
> +        u32 insn;
> +        int i, nr_inst;
> +
> +        if ( !cpus_have_cap(alt->cpufeature) )
> +            continue;
> +
> +        BUG_ON(alt->alt_len != alt->orig_len);
> +
> +        origptr = ALT_ORIG_PTR(alt);
> +        writeptr = origptr - (u32 *)_start + writemap;
> +        replptr = ALT_REPL_PTR(alt);
> +
> +        nr_inst = alt->alt_len / sizeof(insn);
> +
> +        for ( i = 0; i < nr_inst; i++ )
> +        {
> +            insn = get_alt_insn(alt, origptr + i, replptr + i);
> +            *(writeptr + i) = cpu_to_le32(insn);
> +        }
> +
> +        /* Ensure the new instructions reached the memory and nuke */
> +        clean_and_invalidate_dcache_va_range(writeptr,
> +                                             (sizeof (*writeptr) * nr_inst));
> +    }
> +
> +    /* Nuke the instruction cache */
> +    invalidate_icache();
> +
> +    vunmap(writemap);
> +
> +    return 0;
> +}
> +
> +/*
> + * We might be patching the stop_machine state machine, so implement a
> + * really simple polling protocol here.
> + */
> +static int __apply_alternatives_multi_stop(void *unused)
> +{
> +    static int patched = 0;
> +    struct alt_region region = {
> +        .begin = __alt_instructions,
> +        .end = __alt_instructions_end,
> +    };
> +
> +    /* We always have a CPU 0 at this point (__init) */
> +    if ( smp_processor_id() )
> +    {
> +        while ( !read_atomic(&patched) )
> +            cpu_relax();
> +        isb();
> +    }
> +    else
> +    {
> +        int ret;
> +
> +        BUG_ON(patched);
> +        ret = __apply_alternatives(&region);
> +        /* The patching is not expected to fail during boot. */
> +        BUG_ON(ret != 0);
> +
> +        /* Barriers provided by the cache flushing */
> +        write_atomic(&patched, 1);
> +    }
> +
> +    return 0;
> +}
> +
> +/*
> + * This function should only be called during boot and before CPU0 jump
> + * into the idle_loop.
> + */
> +void __init apply_alternatives_all(void)
> +{
> +    int ret;
> +
> +     /* better not try code patching on a live SMP system */
> +    ret = stop_machine_run(__apply_alternatives_multi_stop, NULL, NR_CPUS);
> +
> +    /* stop_machine_run should never fail at this stage of the boot */
> +    BUG_ON(ret);
> +}
> +
> +int apply_alternatives(void *start, size_t length)
> +{
> +    struct alt_region region = {
> +        .begin = start,
> +        .end = start + length,
> +    };
> +
> +    return __apply_alternatives(&region);
> +}
> +
> +/*
> + * Local variables:
> + * mode: C
> + * c-file-style: "BSD"
> + * c-basic-offset: 4
> + * indent-tabs-mode: nil
> + * End:
> + */
> diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
> index 46cf6dd..97b3214 100644
> --- a/xen/arch/arm/setup.c
> +++ b/xen/arch/arm/setup.c
> @@ -38,6 +38,7 @@
>  #include <xen/vmap.h>
>  #include <xen/libfdt/libfdt.h>
>  #include <xen/acpi.h>
> +#include <asm/alternative.h>
>  #include <asm/page.h>
>  #include <asm/current.h>
>  #include <asm/setup.h>
> @@ -835,6 +836,12 @@ void __init start_xen(unsigned long boot_phys_offset,
>  
>      do_initcalls();
>  
> +    /*
> +     * It needs to be called after do_initcalls to be able to use
> +     * stop_machine (tasklets initialized via an initcall).
> +     */
> +    apply_alternatives_all();
> +
>      /* Create initial domain 0. */
>      /* The vGIC for DOM0 is exactly emulating the hardware GIC */
>      config.gic_version = XEN_DOMCTL_CONFIG_GIC_NATIVE;
> diff --git a/xen/arch/arm/xen.lds.S b/xen/arch/arm/xen.lds.S
> index b18c9c2..b24e93b 100644
> --- a/xen/arch/arm/xen.lds.S
> +++ b/xen/arch/arm/xen.lds.S
> @@ -151,6 +151,15 @@ SECTIONS
>         *(.initcall1.init)
>         __initcall_end = .;
>  
> +#ifdef CONFIG_ALTERNATIVE
> +       . = ALIGN(4);
> +       __alt_instructions = .;
> +       *(.altinstructions)
> +       __alt_instructions_end = .;
> +       . = ALIGN(4);
> +       *(.altinstr_replacement)
> +#endif
> +
>         *(.init.data)
>         *(.init.data.rel)
>         *(.init.data.rel.*)
> diff --git a/xen/include/asm-arm/alternative.h 
> b/xen/include/asm-arm/alternative.h
> new file mode 100644
> index 0000000..4287bac
> --- /dev/null
> +++ b/xen/include/asm-arm/alternative.h
> @@ -0,0 +1,165 @@
> +#ifndef __ASM_ALTERNATIVE_H
> +#define __ASM_ALTERNATIVE_H
> +
> +#include <asm/cpufeature.h>
> +#include <xen/config.h>
> +#include <xen/kconfig.h>
> +
> +#ifdef CONFIG_ALTERNATIVE
> +
> +#ifndef __ASSEMBLY__
> +
> +#include <xen/init.h>
> +#include <xen/types.h>
> +#include <xen/stringify.h>
> +
> +struct alt_instr {
> +     s32 orig_offset;        /* offset to original instruction */
> +     s32 alt_offset;         /* offset to replacement instruction */
> +     u16 cpufeature;         /* cpufeature bit set for replacement */
> +     u8  orig_len;           /* size of original instruction(s) */
> +     u8  alt_len;            /* size of new instruction(s), <= orig_len */
> +};
> +
> +void __init apply_alternatives_all(void);
> +int apply_alternatives(void *start, size_t length);
> +
> +#define ALTINSTR_ENTRY(feature)                                              
>       \
> +     " .word 661b - .\n"                             /* label           */ \
> +     " .word 663f - .\n"                             /* new instruction */ \
> +     " .hword " __stringify(feature) "\n"            /* feature bit     */ \
> +     " .byte 662b-661b\n"                            /* source len      */ \
> +     " .byte 664f-663f\n"                            /* replacement len */
> +
> +/*
> + * alternative assembly primitive:
> + *
> + * If any of these .org directive fail, it means that insn1 and insn2
> + * don't have the same length. This used to be written as
> + *
> + * .if ((664b-663b) != (662b-661b))
> + *   .error "Alternatives instruction length mismatch"
> + * .endif
> + *
> + * but most assemblers die if insn1 or insn2 have a .inst. This should
> + * be fixed in a binutils release posterior to 2.25.51.0.2 (anything
> + * containing commit 4e4d08cf7399b606 or c1baaddf8861).
> + */
> +#define __ALTERNATIVE_CFG(oldinstr, newinstr, feature, cfg_enabled)  \
> +     ".if "__stringify(cfg_enabled)" == 1\n"                         \
> +     "661:\n\t"                                                      \
> +     oldinstr "\n"                                                   \
> +     "662:\n"                                                        \
> +     ".pushsection .altinstructions,\"a\"\n"                         \
> +     ALTINSTR_ENTRY(feature)                                         \
> +     ".popsection\n"                                                 \
> +     ".pushsection .altinstr_replacement, \"a\"\n"                   \
> +     "663:\n\t"                                                      \
> +     newinstr "\n"                                                   \
> +     "664:\n\t"                                                      \
> +     ".popsection\n\t"                                               \
> +     ".org   . - (664b-663b) + (662b-661b)\n\t"                      \
> +     ".org   . - (662b-661b) + (664b-663b)\n"                        \
> +     ".endif\n"
> +
> +#define _ALTERNATIVE_CFG(oldinstr, newinstr, feature, cfg, ...)      \
> +     __ALTERNATIVE_CFG(oldinstr, newinstr, feature, IS_ENABLED(cfg))
> +
> +#else
> +
> +#include <asm/asm_defns.h>
> +
> +.macro altinstruction_entry orig_offset alt_offset feature orig_len alt_len
> +     .word \orig_offset - .
> +     .word \alt_offset - .
> +     .hword \feature
> +     .byte \orig_len
> +     .byte \alt_len
> +.endm
> +
> +.macro alternative_insn insn1, insn2, cap, enable = 1
> +     .if \enable
> +661: \insn1
> +662: .pushsection .altinstructions, "a"
> +     altinstruction_entry 661b, 663f, \cap, 662b-661b, 664f-663f
> +     .popsection
> +     .pushsection .altinstr_replacement, "ax"
> +663: \insn2
> +664: .popsection
> +     .org    . - (664b-663b) + (662b-661b)
> +     .org    . - (662b-661b) + (664b-663b)
> +     .endif
> +.endm
> +
> +/*
> + * Begin an alternative code sequence.
> + *
> + * The code that follows this macro will be assembled and linked as
> + * normal. There are no restrictions on this code.
> + */
> +.macro alternative_if_not cap, enable = 1
> +     .if \enable
> +     .pushsection .altinstructions, "a"
> +     altinstruction_entry 661f, 663f, \cap, 662f-661f, 664f-663f
> +     .popsection
> +661:
> +     .endif
> +.endm
> +
> +/*
> + * Provide the alternative code sequence.
> + *
> + * The code that follows this macro is assembled into a special
> + * section to be used for dynamic patching. Code that follows this
> + * macro must:
> + *
> + * 1. Be exactly the same length (in bytes) as the default code
> + *    sequence.
> + *
> + * 2. Not contain a branch target that is used outside of the
> + *    alternative sequence it is defined in (branches into an
> + *    alternative sequence are not fixed up).
> + */
> +.macro alternative_else
> +662: .pushsection .altinstr_replacement, "ax"
> +663:
> +.endm
> +
> +/*
> + * Complete an alternative code sequence.
> + */
> +.macro alternative_endif
> +664: .popsection
> +     .org    . - (664b-663b) + (662b-661b)
> +     .org    . - (662b-661b) + (664b-663b)
> +.endm
> +
> +#define _ALTERNATIVE_CFG(insn1, insn2, cap, cfg, ...)        \
> +     alternative_insn insn1, insn2, cap, IS_ENABLED(cfg)
> +
> +#endif  /*  __ASSEMBLY__  */
> +
> +/*
> + * Usage: asm(ALTERNATIVE(oldinstr, newinstr, feature));
> + *
> + * Usage: asm(ALTERNATIVE(oldinstr, newinstr, feature, CONFIG_FOO));
> + * N.B. If CONFIG_FOO is specified, but not selected, the whole block
> + *      will be omitted, including oldinstr.
> + */
> +#define ALTERNATIVE(oldinstr, newinstr, ...)   \
> +     _ALTERNATIVE_CFG(oldinstr, newinstr, __VA_ARGS__, 1)
> +
> +#else /* !CONFIG_ALTERNATIVE */
> +
> +static inline void apply_alternatives_all(void)
> +{
> +}
> +
> +static inline int apply_alternatives(void *start, size_t lenght)
> +{
> +    return 0;
> +}
> +
> +#endif
> +
> +#endif /* __ASM_ALTERNATIVE_H */
> diff --git a/xen/include/asm-arm/arm64/insn.h 
> b/xen/include/asm-arm/arm64/insn.h
> index cfcdbe9..6ce37be 100644
> --- a/xen/include/asm-arm/arm64/insn.h
> +++ b/xen/include/asm-arm/arm64/insn.h
> @@ -61,6 +61,22 @@ u32 aarch64_insn_encode_immediate(enum 
> aarch64_insn_imm_type type,
>  s32 aarch64_get_branch_offset(u32 insn);
>  u32 aarch64_set_branch_offset(u32 insn, s32 offset);
>  
> +/* Wrapper for common code */
> +static inline bool insn_is_branch_imm(u32 insn)
> +{
> +    return aarch64_insn_is_branch_imm(insn);
> +}
> +
> +static inline s32 insn_get_branch_offset(u32 insn)
> +{
> +    return aarch64_get_branch_offset(insn);
> +}
> +
> +static inline u32 insn_set_branch_offset(u32 insn, s32 offset)
> +{
> +    return aarch64_set_branch_offset(insn, offset);
> +}
> +
>  #endif /* !__ARCH_ARM_ARM64_INSN */
>  /*
>   * Local variables:
> -- 
> 1.9.1
> 

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