Re: [RFC v2-fix-v2 2/2] x86/tdx: Ignore WBINVD instruction for TDX guest

[Kuppuswamy, Sathyanarayanan]

On 5/24/21 4:39 PM, Dan Williams wrote:
> > Functionally only DMA devices can notice a side effect from
> > WBINVD's cache flushing. But, TDX does not support DMA,
> > because DMA typically needs uncached access for MMIO, and
> > the current TDX module always sets the IgnorePAT bit, which
> > prevents that.

> I thought we discussed that there are other considerations for wbinvd
> besides DMA? In any event this paragraph is actively misleading
> because it disregards ACPI and Persistent Memory secure-erase whose
> usages of wbinvd have nothing to do with DMA. I would much prefer a
> patch to shutdown all the known wbinvd users as a precursor to this
> patch rather than assuming it's ok to simply ignore it. You have
> mentioned that TDX does not need to use those paths, but rather than
> assume they can't be used why not do the audit to explicitly disable
> them? Otherwise this statement seems to imply that the audit has not
> been done.

But KVM also emulates WBINVD only if DMA is supported. Otherwise it
will be treated as noop.

static bool need_emulate_wbinvd(struct kvm_vcpu *vcpu)
{
        return kvm_arch_has_noncoherent_dma(vcpu->kvm);
}



--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer