Re: [Xen-devel] [PATCH 10/13] xen/arm64: Implement a fast path for handling SMCCC_ARCH_WORKAROUND_2

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 22 May 2018, Julien Grall wrote:
> The function ARM_SMCCC_ARCH_WORKAROUND_2 will be called by the guest for
> enabling/disabling the ssbd mitigation. So we want the handling to
> be as fast as possible.
> 
> The new sequence will forward guest's ARCH_WORKAROUND_2 call to EL3 and
> also track the state of the workaround per-vCPU.
> 
> Note that since we need to execute branches, this always executes after
> the spectre-v2 mitigation.
> 
> This code is based on KVM counterpart "arm64: KVM: Handle guest's
> ARCH_WORKAROUND_2 requests" written by Marc Zyngier.
> 
> This is part of XSA-263.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

I think the patch works as intended.


> ---
>  xen/arch/arm/arm64/asm-offsets.c |  2 ++
>  xen/arch/arm/arm64/entry.S       | 43 
> +++++++++++++++++++++++++++++++++++++++-
>  xen/arch/arm/cpuerrata.c         | 18 +++++++++++++++++
>  3 files changed, 62 insertions(+), 1 deletion(-)
> 
> diff --git a/xen/arch/arm/arm64/asm-offsets.c 
> b/xen/arch/arm/arm64/asm-offsets.c
> index ce24e44473..f5c696d092 100644
> --- a/xen/arch/arm/arm64/asm-offsets.c
> +++ b/xen/arch/arm/arm64/asm-offsets.c
> @@ -22,6 +22,7 @@
>  void __dummy__(void)
>  {
>     OFFSET(UREGS_X0, struct cpu_user_regs, x0);
> +   OFFSET(UREGS_X1, struct cpu_user_regs, x1);
>     OFFSET(UREGS_LR, struct cpu_user_regs, lr);
>  
>     OFFSET(UREGS_SP, struct cpu_user_regs, sp);
> @@ -45,6 +46,7 @@ void __dummy__(void)
>     BLANK();
>  
>     DEFINE(CPUINFO_sizeof, sizeof(struct cpu_info));
> +   OFFSET(CPUINFO_flags, struct cpu_info, flags);
>  
>     OFFSET(VCPU_arch_saved_context, struct vcpu, arch.saved_context);
>  
> diff --git a/xen/arch/arm/arm64/entry.S b/xen/arch/arm/arm64/entry.S
> index e2344e565f..8e25ff3997 100644
> --- a/xen/arch/arm/arm64/entry.S
> +++ b/xen/arch/arm/arm64/entry.S
> @@ -1,4 +1,6 @@
>  #include <asm/asm_defns.h>
> +#include <asm/current.h>
> +#include <asm/macros.h>
>  #include <asm/regs.h>
>  #include <asm/alternative.h>
>  #include <asm/smccc.h>
> @@ -241,7 +243,7 @@ guest_sync:
>           * be encoded as an immediate for cmp.
>           */
>          eor     w0, w0, #ARM_SMCCC_ARCH_WORKAROUND_1_FID
> -        cbnz    w0, guest_sync_slowpath
> +        cbnz    w0, check_wa2
>  
>          /*
>           * Clobber both x0 and x1 to prevent leakage. Note that thanks
> @@ -250,6 +252,45 @@ guest_sync:
>          mov     x1, xzr
>          eret
>  
> +check_wa2:
> +        /* ARM_SMCCC_ARCH_WORKAROUND_2 handling */
> +        eor     w0, w0, #ARM_SMCCC_ARCH_WORKAROUND_1_FID

We come to check_wa2 after checking on #ARM_SMCCC_ARCH_WORKAROUND_1_FID,
so maybe we can skip this?


> +        eor     w0, w0, #ARM_SMCCC_ARCH_WORKAROUND_2_FID
> +        cbnz    w0, guest_sync_slowpath
> +#ifdef CONFIG_ARM_SSBD
> +alternative_cb arm_enable_wa2_handling
> +        b       wa2_end
> +alternative_cb_end
> +        /* Sanitize the argument */
> +        mov     x0, #-(UREGS_kernel_sizeof - UREGS_X1)  /* x0 := offset of 
> guest's x1 on the stack */
> +        ldr     x1, [sp, x0]                            /* Load guest's x1 */
> +        cmp     w1, wzr
> +        cset    x1, ne
> +
> +        /*
> +         * Update the guest flag. At this stage sp point after the field
> +         * guest_cpu_user_regs in cpu_info.
> +         */
> +        adr_cpu_info x2
> +        ldr     x0, [x2, #CPUINFO_flags]
> +        bfi     x0, x1, #CPUINFO_WORKAROUND_2_FLAG_SHIFT, #1
> +        str     x0, [x2, #CPUINFO_flags]
> +
> +        /* Check that we actually need to perform the call */
> +        ldr_this_cpu x0, ssbd_callback_required, x2
> +        cbz     x0, wa2_end
> +        mov     w0, #ARM_SMCCC_ARCH_WORKAROUND_2_FID
> +        smc     #0

Shouldn't we make the call only if get_cpu_info()->flags changed?


> +wa2_end:
> +        /* Don't leak data from the SMC call */
> +        mov     x1, xzr
> +        mov     x2, xzr
> +        mov     x3, xzr
> +#endif /* !CONFIG_ARM_SSBD */
> +        mov     x0, xzr
> +        eret
>  guest_sync_slowpath:
>          /*
>           * x0/x1 may have been scratch by the fast path above, so avoid
> diff --git a/xen/arch/arm/cpuerrata.c b/xen/arch/arm/cpuerrata.c
> index f921721a66..54df4ff445 100644
> --- a/xen/arch/arm/cpuerrata.c
> +++ b/xen/arch/arm/cpuerrata.c
> @@ -7,6 +7,7 @@
>  #include <xen/warning.h>
>  #include <asm/cpufeature.h>
>  #include <asm/cpuerrata.h>
> +#include <asm/insn.h>
>  #include <asm/psci.h>
>  
>  /* Override macros from asm/page.h to make them work with mfn_t */
> @@ -272,6 +273,23 @@ static int __init parse_spec_ctrl(const char *s)
>  }
>  custom_param("spec-ctrl", parse_spec_ctrl);
>  
> +/* Arm64 only for now as for Arm32 the workaround is currently handled in C. 
> */
> +#ifdef CONFIG_ARM_64
> +void __init arm_enable_wa2_handling(const struct alt_instr *alt,
> +                                    const uint32_t *origptr,
> +                                    uint32_t *updptr, int nr_inst)
> +{
> +    BUG_ON(nr_inst != 1);
> +
> +    /*
> +     * Only allow mitigation on guest ARCH_WORKAROUND_2 if the SSBD
> +     * state allow it to be flipped.
> +     */
> +    if ( get_ssbd_state() == ARM_SSBD_RUNTIME )
> +        *updptr = aarch64_insn_gen_nop();
> +}
> +#endif
> +
>  /*
>   * Assembly code may use the variable directly, so we need to make sure
>   * it fits in a register.
> -- 
> 2.11.0
> 

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