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Re: [PATCH v1] x86/nSVM: Expose the FlushByASID CPU Capability to L1 guests
- To: Abdelkareem Abdelsaamad <abdelkareem.abdelsaamad@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 13 Aug 2026 17:29:18 +0200
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- Cc: andrew.cooper3@xxxxxxxxxx, jason.andryuk@xxxxxxx, teddy.astie@xxxxxxxxxx, Roger Pau Monné <roger@xxxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 13 Aug 2026 15:29:28 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 13.08.2026 16:56, Abdelkareem Abdelsaamad wrote:
> On the AMD platforms, the Xen hypervisor requires the FlushByASID CPU
> capability to support HVM nested virtualization (see start_nested_svm).
> Consequently, the L1 hypervisor must report FlushByASID CPU capability support
> when intercepting CPUID instruction for the CPU feature from the L2 guest to
> support nested virtualization levels beyond L1. Extend the exposed HVM CPU
> policy to surface this CPU feature support for the guests.
While the change makes sense, I have to admit that I consider it a stretch
to justify changes by multi-level nesting, when a single level of nesting
is in need of a lot of work to actually behave sensibly. Further, "to
support nested virtualization levels beyond L1" looks pretty Xen-centric:
Other hypervisors may permit this without the feature.
> While at it remove the dangling `exitinfo1 = ns_vmcb->exitinfo1;` assignment
> inside the nested exit handling of svm_vmexit_handler().
Unrelated adjustments to somewhat nearby or related code are generally
okay, but here you're touching a different file and entirely unrelated
code. I think the two changes want splitting.
> Signed-off-by: Abdelkareem Abdelsaamad <abdelkareem.abdelsaamad@xxxxxxxxxx>
> ---
> Testing:
> - Using a locally developed XTF test to call cpuid_edx(0x8000000aU);
> - without the change, EDX returns 0x4AB (the FlushByASID 6th bit is not
> set).
> - with the change, EDX returns 0x4EB (the FlushByASID 6th bit set).
And the CPUID test that XTF has wasn't suitable?
Finally: Can you please drop Roger's old email address that you still
had on Cc?
Jan
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