linux-kernel - Re: [RFC PATCH 01/28] x86/pvh: Call C code via the kernel virtual mapping

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Message-ID: <1258d32d-ee54-4a5d-bc92-6c1f8179235a@amd.com>
Date: Wed, 25 Sep 2024 17:12:51 -0400
From: Jason Andryuk <jason.andryuk@....com>
To: Ard Biesheuvel <ardb+git@...gle.com>, <linux-kernel@...r.kernel.org>
CC: Ard Biesheuvel <ardb@...nel.org>, <x86@...nel.org>, "H. Peter Anvin"
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Subject: Re: [RFC PATCH 01/28] x86/pvh: Call C code via the kernel virtual
 mapping

On 2024-09-25 11:01, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@...nel.org>
> 
> 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")
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>

Tested-by: Jason Andryuk <jason.andryuk@....com>
Reviewed-by: Jason Andryuk <jason.andryuk@....com>

I found that before this change xen_prepare_pvh() would call through 
some pv_ops function pointers into the kernel virtual mapping.

Regards,
Jason