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

Jason Andryuk
2024-09-25T21:12:58Z

On 2024-09-25 11:01, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.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@kernel.org>

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