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

Sean Christopherson
2022-08-02T21:15:51Z

On Tue, Aug 02, 2022, Kai Huang wrote:
> On Mon, 2022-08-01 at 23:20 +0000, Sean Christopherson wrote:
> > On Tue, Aug 02, 2022, Kai Huang wrote:
> > > On Mon, 2022-08-01 at 14:15 +0000, Sean Christopherson wrote:
> > > > 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.
> >
> > Right, SUPPRESS_VE isn't strictly necessary, but KVM already deliberately avoids
> > bit 63 because it has meaning, e.g. SUPPRESS_VE for EPT and NX for PAE and 64-bit
> > paging.
> >
> > > I don't see making mask/value both per-vm is a big issue?
> >
> > Yes and no.
> >
> > No, in the sense that it's not a big issue in terms of code.
> >
> > Yes, because of the connotations of having a per-VM mask. While having SUPPRESS_VE
> > in the mask for non-TDX EPT isn't strictly necessary, it's also not strictly necessary
> > to _not_ have it in the mask.
> >
>
> I think the 'mask' itself is ambiguous, i.e. it doesn't say in what circumstance
> we should include one bit to the mask. My understanding is any bit in the
> 'mask' should at least be related to the 'value' that can enable MMIO caching.

The purpose of the mask isn't ambiguous, though it's definitely not well documented.
The mask defines what bits should be included in the check to identify an MMIO SPTE.

> So if SUPPRESS_VE bit is not related to non-TDX EPT (as we want EPT
> misconfiguration, but not EPT violation), I don't see why we need to include it
> to the 'mask'.

Again, it's not strictly necessary, but by doing so we don't need a per-VM mask.
And KVM should also never set SUPPRESS_VE for MMIO SPTEs, i.e. checking that bit
by including it in the mask adds some sanitcy check (albeit a miniscule amount).

> > In other words, having a per-VM mask incorrectly implies that TDX _must_
> > have a different mask.
>
> I interpret as TDX _can_, but not _must_.

Right, but if we write the KVM code such that it doesn't have a different mask,
then even that "can" is wrong/misleading.

> > It's also one more piece of information that developers have to track down and
> > account for, i.e. one more thing we can screw up.
> >
> > The other aspect of MMIO SPTEs are that the mask bits must not overlap the generation
> > bits or shadow-present bit, and changing any of those bits requires careful
> > consideration, i.e. defining the set of _allowed_ mask bits on a per-VM basis would
> > incur significant complexity without providing meaningful benefit.
> >
>
> Agreed on this.
>
> But we are not checking any of those in kvm_mmu_set_mmio_spte_mask(), right? :)

No, but we really should.

> Also Isaku's patch extends kvm_mmu_set_mmio_spte_mask() to take 'kvm' or 'vcpu'
> as parameter so it's easy to check there -- not 100% sure about other places,
> though.
>
> > As a result,
> > it's highly unlikely that we'll ever want to opportunsitically "reclaim" bit 63
> > for MMIO SPTEs, so there's practically zero cost if it's included in the mask for
> > non-TDX EPT.
>
> Sorry I don't understand this. If we will never "reclaim" bit 63 for MMIO SPTEs
> (for non-TDX EPT), then why bother including it to the mask?

Because then we don't need to track a per-VM mask.