Re: [PATCH 4/4] KVM: SVM: Disable SEV-ES support if MMIO caching is disable

Kai Huang
2022-08-01T09:30:46Z

On Fri, 2022-07-29 at 15:21 +0000, Sean Christopherson wrote:
> On Fri, Jul 29, 2022, Kai Huang wrote:
> > On Thu, 2022-07-28 at 22:17 +0000, Sean Christopherson wrote:
> > > Disable SEV-ES if MMIO caching is disabled as SEV-ES relies on MMIO SPTEs
> > > generating #NPF(RSVD), which are reflected by the CPU into the guest as
> > > a #VC. With SEV-ES, the untrusted host, a.k.a. KVM, doesn't have access
> > > to the guest instruction stream or register state and so can't directly
> > > emulate in response to a #NPF on an emulated MMIO GPA. Disabling MMIO
> > > caching means guest accesses to emulated MMIO ranges cause #NPF(!PRESENT),
> > > and those flavors of #NPF cause automatic VM-Exits, not #VC.
> > >
> > > Fixes: b09763da4dd8 ("KVM: x86/mmu: Add module param to disable MMIO caching (for testing)")
> > > Reported-by: Michael Roth <michael.roth@amd.com>
> > > Cc: Tom Lendacky <thomas.lendacky@amd.com>
> > > Cc: stable@vger.kernel.org
> > > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > > ---
>
> ...
>
> > > + /*
> > > + * SEV-ES requires MMIO caching as KVM doesn't have access to the guest
> > > + * instruction stream, i.e. can't emulate in response to a #NPF and
> > > + * instead relies on #NPF(RSVD) being reflected into the guest as #VC
> > > + * (the guest can then do a #VMGEXIT to request MMIO emulation).
> > > + */
> > > + if (!enable_mmio_caching)
> > > + goto out;
> > > +
> > >
> >
> > I am not familiar with SEV, but looks it is similar to TDX -- they both causes
> > #VE to guest instead of faulting into KVM.  And they both require explicit call
> > from guest to do MMIO.
> >
> > In this case, does existing MMIO caching logic still apply to them?
>
> Yes, because TDX/SEV-ES+ need to generate #VE/#VC on emulated MMIO so that legacy
> (or intentionally unenlightened) software in the guest doesn't simply hang/die on
> memory accesses to emulated MMIO (as opposed to direct TDVMCALL/#VMGEXIT).
>
> > Should we still treat SEV and TDX's MMIO handling as MMIO caching being
> > enabled? Or perhaps another variable?
>
> I don't think a separate variable is necesary. At its core, KVM is still caching
> MMIO GPAs via magic SPTE values. The fact that it's required for functionality
> doesn't make the name wrong.

OK.

>
> SEV-ES+ in particular doesn't have a strong guarantee that inducing #VC via #NPF(RSVD)
> is always possible. Theoretically, an SEV-ES+ capable CPU could ship with an effective
> MAXPHYADDR=51 (after reducing the raw MAXPHYADDR) and C-bit=51, in which case there are
> no resered PA bits and thus no reserved PTE bits at all. That's obviously unlikely to
> happen, but if it does come up, then disabling SEV-ES+ due to MMIO caching not being
> possible is the desired behavior, e.g. either the CPU configuration is bad or KVM is
> lacking support for a newfangled way to support emulated MMIO (in a future theoretical
> SEV-* product).

I bet AMD will see this (your) response and never ship such chips :)

--
Thanks,
-Kai