Re: [RFC PATCH] x86/xen: Fix PVH dom0 xen_hypercall detection

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 10/04/2025 8:50 pm, Jason Andryuk wrote:
> A Xen PVH dom0 on an AMD processor triple faults early in boot on
> 6.6.86.  CPU detection appears to fail, as the faulting instruction is
> vmcall in xen_hypercall_intel() and not vmmcall in xen_hypercall_amd().
>
> Detection fails because __xen_hypercall_setfunc() returns the full
> kernel mapped address of xen_hypercall_amd() or xen_hypercall_intel() -
> e.g. 0xffffffff815b93f0.  But this is compared against the rip-relative
> xen_hypercall_amd(%rip), which when running from identity mapping, is
> only 0x015b93f0.
>
> Replace the rip-relative address with just loading the actual address to
> restore the proper comparision.
>
> This only seems to affect PVH dom0 boot.  This is probably because the
> XENMEM_memory_map hypercall is issued early on from the identity
> mappings.  With a domU, the memory map is provided via hvm_start_info
> and the hypercall is skipped.  The domU is probably running from the
> kernel high mapping when it issues hypercalls.
>
> Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
> ---
> I think this sort of address mismatch would be addresed by
> e8fbc0d9cab6 ("x86/pvh: Call C code via the kernel virtual mapping")
>
> That could be backported instead, but it depends on a fair number of
> patches.

I've just spoken to Ard, and he thinks that it's standalone.  Should be
ok to backport as a fix.

> Not sure on how getting a patch just into 6.6 would work.  This patch
> could go into upstream Linux though it's not strictly necessary when the
> rip-relative address is a high address.

Do we know which other trees are broken?  I only found 6.6 because I was
messing around with other bits of CI that happen to use 6.6.

~Andrew