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

[Sean Christopherson]

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@xxxxxxx>
> > Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > ---

...

> > +	/*
> > +	 * 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.

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).