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x86/pvh: Make PVH entrypoint PIC for x86-64

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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@amd.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Message-ID: <20240823193630.2583107-3-jason.andryuk@amd.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
This commit is contained in:
Jason Andryuk 2024-08-23 15:36:27 -04:00 committed by Juergen Gross
parent 08377ed24f
commit 1db29f99ed
1 changed files with 34 additions and 12 deletions
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46
arch/x86/platform/pvh/head.S
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@ -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)