[PATCH v2 2/5] x86/pvh: Make PVH entrypoint PIC for x86-64

[Jason Andryuk <jason.andryuk@xxxxxxx>]

The PVH entrypoint is 32bit non-PIC code running the uncompressed
vmlinux at its load address CONFIG_PHYSICAL_START - default 0x1000000
(16MB).  The kernel is loaded at that physical address inside the VM by
the VMM software (Xen/QEMU).

When running a Xen PVH Dom0, the host reserved addresses are mapped 1-1
into the PVH container.  There exist system firmwares (Coreboot/EDK2)
with reserved memory at 16MB.  This creates a conflict where the PVH
kernel cannot be loaded at that address.

Modify the PVH entrypoint to be position-indepedent to allow flexibility
in load address.  Only the 64bit entry path is converted.  A 32bit
kernel is not PIC, so calling into other parts of the kernel, like
xen_prepare_pvh() and mk_pgtable_32(), don't work properly when
relocated.

This makes the code PIC, but the page tables need to be updated as well
to handle running from the kernel high map.

The UNWIND_HINT_END_OF_STACK is to silence:
vmlinux.o: warning: objtool: pvh_start_xen+0x7f: unreachable instruction
after the lret into 64bit code.

Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
---
v2:
Use rip-relative loads in 64bit mode
Direct jmp to startup_64
---
 arch/x86/platform/pvh/head.S | 46 ++++++++++++++++++++++++++----------
 1 file changed, 34 insertions(+), 12 deletions(-)

diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
index f7235ef87bc3..ba4d0eab4436 100644
--- a/arch/x86/platform/pvh/head.S
+++ b/arch/x86/platform/pvh/head.S
@@ -7,6 +7,7 @@
        .code32
        .text
 #define _pa(x)          ((x) - __START_KERNEL_map)
+#define rva(x)          ((x) - pvh_start_xen)
 
 #include <linux/elfnote.h>
 #include <linux/init.h>
@@ -54,7 +55,25 @@ SYM_CODE_START_LOCAL(pvh_start_xen)
        UNWIND_HINT_END_OF_STACK
        cld
 
-       lgdt (_pa(gdt))
+       /*
+        * See the comment for startup_32 for more details.  We need to
+        * execute a call to get the execution address to be position
+        * independent, but we don't have a stack.  Save and restore the
+        * magic field of start_info in ebx, and use that as the stack.
+        */
+       mov  (%ebx), %eax
+       leal 4(%ebx), %esp
+       ANNOTATE_INTRA_FUNCTION_CALL
+       call 1f
+1:     popl %ebp
+       mov  %eax, (%ebx)
+       subl $rva(1b), %ebp
+       movl $0, %esp
+
+       leal rva(gdt)(%ebp), %eax
+       leal rva(gdt_start)(%ebp), %ecx
+       movl %ecx, 2(%eax)
+       lgdt (%eax)
 
        mov $PVH_DS_SEL,%eax
        mov %eax,%ds
@@ -62,14 +81,14 @@ SYM_CODE_START_LOCAL(pvh_start_xen)
        mov %eax,%ss
 
        /* Stash hvm_start_info. */
-       mov $_pa(pvh_start_info), %edi
+       leal rva(pvh_start_info)(%ebp), %edi
        mov %ebx, %esi
-       mov _pa(pvh_start_info_sz), %ecx
+       movl rva(pvh_start_info_sz)(%ebp), %ecx
        shr $2,%ecx
        rep
        movsl
 
-       mov $_pa(early_stack_end), %esp
+       leal rva(early_stack_end)(%ebp), %esp
 
        /* Enable PAE mode. */
        mov %cr4, %eax
@@ -84,30 +103,33 @@ SYM_CODE_START_LOCAL(pvh_start_xen)
        wrmsr
 
        /* Enable pre-constructed page tables. */
-       mov $_pa(init_top_pgt), %eax
+       leal rva(init_top_pgt)(%ebp), %eax
        mov %eax, %cr3
        mov $(X86_CR0_PG | X86_CR0_PE), %eax
        mov %eax, %cr0
 
        /* Jump to 64-bit mode. */
-       ljmp $PVH_CS_SEL, $_pa(1f)
+       pushl $PVH_CS_SEL
+       leal  rva(1f)(%ebp), %eax
+       pushl %eax
+       lretl
 
        /* 64-bit entry point. */
        .code64
 1:
+       UNWIND_HINT_END_OF_STACK
+
        /* Set base address in stack canary descriptor. */
        mov $MSR_GS_BASE,%ecx
-       mov $_pa(canary), %eax
+       leal canary(%rip), %eax
        xor %edx, %edx
        wrmsr
 
        call xen_prepare_pvh
 
        /* startup_64 expects boot_params in %rsi. */
-       mov $_pa(pvh_bootparams), %rsi
-       mov $_pa(startup_64), %rax
-       ANNOTATE_RETPOLINE_SAFE
-       jmp *%rax
+       lea pvh_bootparams(%rip), %rsi
+       jmp startup_64
 
 #else /* CONFIG_X86_64 */
 
@@ -143,7 +165,7 @@ SYM_CODE_END(pvh_start_xen)
        .balign 8
 SYM_DATA_START_LOCAL(gdt)
        .word gdt_end - gdt_start
-       .long _pa(gdt_start)
+       .long _pa(gdt_start) /* x86-64 will overwrite if relocated. */
        .word 0
 SYM_DATA_END(gdt)
 SYM_DATA_START_LOCAL(gdt_start)
-- 
2.34.1