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Re: [PATCH 1/5] x86/pvh: Call C code via the kernel virtual mapping
- To: Ard Biesheuvel <ardb@xxxxxxxxxx>, Ard Biesheuvel <ardb+git@xxxxxxxxxx>
- From: Jason Andryuk <jason.andryuk@xxxxxxx>
- Date: Thu, 26 Sep 2024 16:29:54 -0400
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- Cc: <linux-kernel@xxxxxxxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, <x86@xxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 26 Sep 2024 20:31:50 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 2024-09-26 06:55, Ard Biesheuvel wrote:
On Thu, 26 Sept 2024 at 12:41, Ard Biesheuvel <ardb+git@xxxxxxxxxx> wrote:
From: Ard Biesheuvel <ardb@xxxxxxxxxx>
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 <jason.andryuk@xxxxxxx>
Reviewed-by: Jason Andryuk <jason.andryuk@xxxxxxx>
Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
---
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
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