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Re: [PATCH] x86/svm: Always sync guest CR2 on VMExit
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 4 May 2026 07:14:08 +0200
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- Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>
- Delivery-date: Mon, 04 May 2026 05:14:30 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 02.05.2026 00:21, Andrew Cooper wrote:
> On 01/05/2026 10:44 pm, Stefano Stabellini wrote:
>> On Fri, 1 May 2026, Andrew Cooper wrote:
>>> Under SVM, there are two copies of guest CR2. One is
>>> v->arch.hvm.guest_cr[2]
>>> and one is in the VMCB.
>>>
>>> Xen doesn't intercept CR2 accesses, so this mostly goes unnoticed; hardware
>>> loads and saves the guest CR2 across VMRUN/VMExit.
>>>
>>> For HAP guests (where #PF is not intercepted, and therefore we don't
>>> typically
>>> inject #PF either), this causes the guest CR2 value to be lost on migrate.
>>> As
>>> migration is cooperative and not done from the #PF handler, this also goes
>>> unoticed by guests.
>>>
>>> It also means that an emulated MOV-from-CR2 reads a stale value.
>>>
>>> Reported-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>>> Fixes: d1bd157fbc9b ("Big merge the HVM full-virtualisation abstractions.")
>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Tested-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>
> Thanks, sadly I made the mistake of extending my XTF PoC for this.
>
> There are also bugs on the emulated MOV-to-CR2 side, and they're far
> harder to untangle.
Any slightly closer details as to what?
> This patch probably wants to go in in this form, accepting that there
> are issues still to be addressed on the other side.
I agree:
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Jan
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