Re: [PATCH] KVM: x86: Track supported ARCH_CAPABILITIES in kvm_caps

[Xiaoyao Li]

On 5/23/2023 1:43 AM, Sean Christopherson wrote:
> > >    6. Performance aside, KVM should not be speculating (ha!) on what the guest
> > >       will and will not do, and should instead honor whatever behavior is presented
> > >       to the guest.  If the guest CPU model indicates that VERW flushes buffers,
> > >       then KVM damn well needs to let VERW flush buffers.
> > The current implementation allows guests to have VERW flush buffers when
> > they enumerate FB_CLEAR. It only restricts the flush behavior when the
> > guest is trying to mitigate against a vulnerability(like MDS) on a
> > hardware that is not affected. I guess its common for guests to be
> > running with older gen configuration on a newer hardware.
> Right, I'm saying that that behavior is wrong.  KVM shouldn't assume the guest
> the guest will do things a certain way and should instead honor the "architectural"
> definition, in quotes because I realize there probably is no architectural
> definition for any of this.
>
> It might be that the code does (unintentionally?) honor the "architecture", i.e.
> this code might actually be accurrate with respect to when the guest can expect
> VERW to flush buffers.  But the comment is so, so wrong.

The comment is wrong and the code is wrong in some case as well.

If none of ARCH_CAP_FB_CLEAR, ARCH_CAP_MDS_NO, ARCH_CAP_TAA_NO, 
ARCH_CAP_PSDP_NO, ARCH_CAP_FBSDP_NO and ARCH_CAP_SBDR_SSDP_NO are 
exposed to VM, the VM is type of "affected by MDS".

And accroding to the page 
https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/processor-mmio-stale-data-vulnerabilities.html

if the VM enumerates support for both L1D_FLUSH and MD_CLEAR, it 
implicitly enumerates FB_CLEAR as part of their MD_CLEAR support.

However, the code will leave vmx->disable_fb_clear as 1 if hardware 
supports it, and VERW intruction doesn't clear FB in the VM, which 
conflicts "architectural" definition.

> 	/*
> 	 * If guest will not execute VERW, there is no need to set FB_CLEAR_DIS
> 	 * at VMEntry. Skip the MSR read/write when a guest has no use case to
> 	 * execute VERW.
> 	 */
> 	if ((vcpu->arch.arch_capabilities & ARCH_CAP_FB_CLEAR) ||
> 	   ((vcpu->arch.arch_capabilities & ARCH_CAP_MDS_NO) &&
> 	    (vcpu->arch.arch_capabilities & ARCH_CAP_TAA_NO) &&
> 	    (vcpu->arch.arch_capabilities & ARCH_CAP_PSDP_NO) &&
> 	    (vcpu->arch.arch_capabilities & ARCH_CAP_FBSDP_NO) &&
> 	    (vcpu->arch.arch_capabilities & ARCH_CAP_SBDR_SSDP_NO)))
> 		vmx->disable_fb_clear = false;