Commit 1db29f99 authored by Jason Andryuk's avatar Jason Andryuk Committed by Juergen Gross
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x86/pvh: Make PVH entrypoint PIC for x86-64 



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: default avatarJason Andryuk <jason.andryuk@amd.com>
Reviewed-by: default avatarJuergen Gross <jgross@suse.com>
Message-ID: <20240823193630.2583107-3-jason.andryuk@amd.com>
Signed-off-by: default avatarJuergen Gross <jgross@suse.com>
parent 08377ed2

arch/x86/platform/pvh/head.S

+34 −12
Original line number Diff line number Diff line
@@ -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)