Re: [PATCH] xen/x86/hvm: Propagate emulated CR2 writes to the VMCB

[Ross Lagerwall <ross.lagerwall@citrix.com>]

On 10/21/26 9:19 AM, Roger Pau Monné wrote:
> On Tue, Oct 20, 2026 at 02:33:15PM -0700, Stefano Stabellini wrote:
>> hvmemul_write_cr() updates guest_cr[2] but, unlike every other writer of
>> it, doesn't call hvm_update_guest_cr(). On SVM, CR2 is loaded from
>> the VMCB, so the emulated write never reaches the guest, and the next
>> VMExit overwrites guest_cr[2] with the stale VMCB value.
>>
>> Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
> 
> Acked-by: Roger Pau Monné <roger@xenproject.org>
> 
> I wonder whether there's more missing here, see below.
> 
>> ---
>>   xen/arch/x86/hvm/emulate.c | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
>> index c09ea002ec..cd2831d34b 100644
>> --- a/xen/arch/x86/hvm/emulate.c
>> +++ b/xen/arch/x86/hvm/emulate.c
>> @@ -2350,6 +2350,7 @@ static int cf_check hvmemul_write_cr(
>>   
>>       case 2:
>>           curr->arch.hvm.guest_cr[2] = val;
>> +        hvm_update_guest_cr(curr, 2);
> 
> Looking at hvm_update_cr(), is this possibly also missing a call to
> nestedhvm_set_cr()?

AFAIK the values set by nestedhvm_set_cr() are not used at all with nested SVM
and from a brief look may only be used for CR0 and CR4 for VVMX. So maybe it is
not needed, though for consistency I think it would make sense to replace both
lines with a single call to hvm_update_cr().

Ross