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: 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>

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)