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SubjectRe: [PATCH v4 00/21] AMX Support in KVM
On Tue, Jan 04, 2022, Paolo Bonzini wrote:
> On 12/29/21 14:13, Yang Zhong wrote:
> > Highly appreciate for your review. This version mostly addressed the comments
> > from Sean. Most comments are adopted except three which are not closed and
> > need more discussions:
> >
> > - Move the entire xfd write emulation code to x86.c. Doing so requires
> > introducing a new kvm_x86_ops callback to disable msr write bitmap.
> > According to Paolo's earlier comment he prefers to handle it in vmx.c.
>
> Yes, I do.

No objection, my comments were prior to seeing the patches that manipulated the
bitmap, e.g. in the earlier patches, having anything in vmx.c is unnecessary.

> > - Directly check msr_bitmap in update_exception_bitmap() (for
> > trapping #NM) and vcpu_enter_guest() (for syncing guest xfd after
> > vm-exit) instead of introducing an extra flag in the last patch. However,
> > doing so requires another new kvm_x86_ops callback for checking
> > msr_bitmap since vcpu_enter_guest() is x86 common code. Having an
> > extra flag sounds simpler here (at least for the initial AMX support).
> > It does penalize nested guest with one xfd sync per exit, but it's not
> > worse than a normal guest which initializes xfd but doesn't run
> > AMX applications at all. Those could be improved afterwards.
>
> The thing to do here would be to move
> MAX_POSSIBLE_PASSTHROUGH_MSRS/MAX_DIRECT_ACCESS_MSRS from VMX/SVM to core
> code. For now we can keep the flag.
>
> > - Disable #NM trap for nested guest. This version still chooses to always
> > trap #NM (regardless in L1 or L2) as long as xfd write interception is disabled.
> > In reality #NM is rare if nested guest doesn't intend to run AMX applications
> > and always-trap is safer than dynamic trap for the basic support in case
> > of any oversight here.
>
> Sean was justifying this with lack of support for nested AMX, but I'm not
> sure actually what is missing at all. That is, an L1 hypervisor could
> expose AMX to L2, and then an L2->L0->L2 exit/reentry would have to trap
> #NM. Otherwise it would miss an XFD_ERR update.

Ya, I was assuming there was something L0 needed to do to supported nested AMX,
but as Paolo pointed out there are no VMCS bits, so L0 just needs to correctly
handle #NM and MSR interceptions according to vmcs12.

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