On 2024-09-26 06:55, Ard Biesheuvel wrote:
On Thu, 26 Sept 2024 at 12:41, Ard Biesheuvel <[email protected]> wrote:
From: Ard Biesheuvel <[email protected]>
Calling C code via a different mapping than it was linked at is
problematic, because the compiler assumes that RIP-relative and absolute
symbol references are interchangeable. GCC in particular may use
RIP-relative per-CPU variable references even when not using -fpic.
So call xen_prepare_pvh() via its kernel virtual mapping on x86_64, so
that those RIP-relative references produce the correct values. This
matches the pre-existing behavior for i386, which also invokes
xen_prepare_pvh() via the kernel virtual mapping before invoking
startup_32 with paging disabled again.
Fixes: 7243b93345f7 ("xen/pvh: Bootstrap PVH guest")
Tested-by: Jason Andryuk <[email protected]>
Reviewed-by: Jason Andryuk <[email protected]>
Signed-off-by: Ard Biesheuvel <[email protected]>
---
arch/x86/platform/pvh/head.S | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
index 64fca49cd88f..98ddd552885a 100644
--- a/arch/x86/platform/pvh/head.S
+++ b/arch/x86/platform/pvh/head.S
@@ -172,7 +172,13 @@ SYM_CODE_START_LOCAL(pvh_start_xen)
movq %rbp, %rbx
subq $_pa(pvh_start_xen), %rbx
movq %rbx, phys_base(%rip)
- call xen_prepare_pvh
+
+ /* Call xen_prepare_pvh() via the kernel virtual mapping */
+ leaq xen_prepare_pvh(%rip), %rax
Just realized that we probably need
+ subq phys_base(%rip), %rax
Yes, this is necessary when phys_base is non-0. I intended to test a
non-0 case yesterday, but it turns out I didn't. Re-testing, I have
confirmed this subq is necessary.
Thanks,
Jason
