Re: [PATCH 2/4] KVM: x86/mmu: Fully re-evaluate MMIO caching when SPTE masks change

Kai Huang
2022-08-01T20:54:47Z

On Mon, 2022-08-01 at 14:15 +0000, Sean Christopherson wrote:
> On Mon, Aug 01, 2022, Kai Huang wrote:
> > On Fri, 2022-07-29 at 15:07 +0000, Sean Christopherson wrote:
> > > Lastly, in prepration for TDX, enable_mmio_caching should be changed to key off
> > > of the _mask_, not the value. E.g. for TDX, the value will be '0', but the mask
> > > should be SUPPRESS_VE | RWX.
> >
> > Agreed. But perhaps in another patch. We need to re-define what does
> > mask/value mean to enable_mmio_caching.
>
> There's no need to redefine what they mean, the only change that needs to be made
> is handle the scenario where desire value is '0'. Maybe that's all you mean by
> "redefine"?

My thinking is only when mask and value both are 0, enable_mmio_caching is
considered disabled. vlaue=0 is valid when enable_mmio_caching is true as you
said.

>
> Another thing to note is that only the value needs to be per-VM, the mask can be
> KVM-wide, i.e. "mask = SUPPRESS_VE | RWX" will work for TDX and non-TDX VMs when
> EPT is enabled.

Yeah, but is more like VMX and TDX both *happen* to have the same mask?
Theoretically, VMX only need RWX to trigger EPT misconfiguration but doesn't
need SUPPRESS_VE.

I don't see making mask/value both per-vm is a big issue?

--
Thanks,
-Kai