Re: [PATCH V2 02/12] KVM: x86: Allow the use of kvm_load_host_xsave_state() with guest_state_protected

Sean Christopherson
2025-03-07T23:04:46Z

On Thu, Mar 06, 2025, Paolo Bonzini wrote:
> Il gio 6 mar 2025, 21:44 Sean Christopherson <seanjc@google.com> ha scritto:
> > > Allowing the use of kvm_load_host_xsave_state() is really ugly, especially
> > > since the corresponding code is so simple:
> > >
> > > if (cpu_feature_enabled(X86_FEATURE_PKU) && vcpu->arch.pkru != 0)
> > > wrpkru(vcpu->arch.host_pkru);
> >
> > It's clearly not "so simple", because this code is buggy.
> >
> > The justification for using kvm_load_host_xsave_state() is that either KVM gets
> > the TDX state model correct and the existing flows Just Work, or we handle all
> > that state as one-offs and at best replicate concepts and flows, and at worst
> > have bugs that are unique to TDX, e.g. because we get the "simple" code wrong,
> > we miss flows that subtly consume state, etc.
>
> A typo doesn't change the fact that kvm_load_host_xsave_state is
> optimized with knowledge of the guest CR0 and CR4; faking the values
> so that the same field means both "exit value" and "guest value",

I can't argue against that, but I still absolutely detest carrying dedicated code
for SEV and TDX state management. It's bad enough that figuring out WTF actually
happens basically requires encyclopedic knowledge of massive specs.

I tried to figure out a way to share code, but everything I can come up with that
doesn't fake vCPU state makes the non-TDX code a mess. :-(

> just so that the common code does the right thing for pkru/xcr0/xss,

FWIW, it's not just to that KVM does the right thing for those values, it's a
defense in depth mechanism so that *when*, not if, KVM screws up, the odds of the
bug being fatal to KVM and/or the guest are reduced.

> is > unmaintainable and conceptually just wrong.

I don't necessarily disagree, but what we have today isn't maintainable either.
Without actual sanity check and safeguards in the low level helpers, we absolutely
are playing a game of whack-a-mole.

E.g. see commit 9b42d1e8e4fe ("KVM: x86: Play nice with protected guests in
complete_hypercall_exit()").

At a glance, kvm_hv_hypercall() is still broken, because is_protmode() will return
false incorrectly.

> And while the change for XSS (and possibly other MSRs) is actually correct,
> it should be justified for both SEV-ES/SNP and TDX rather than sneaked into
> the TDX patches.
>
> While there could be other flows that consume guest state, they're
> just as bound to do the wrong thing if vcpu->arch is only guaranteed
> to be somehow plausible (think anything that for whatever reason uses
> cpu_role).

But the MMU code is *already* broken. kvm_init_mmu() => vcpu_to_role_regs(). It
"works" because the fubar role is never truly consumed. I'm sure there are more
examples.

> There's no way the existing flows for !guest_state_protected should run _at
> all_ when the register state is not there. If they do, it's a bug and fixing
> them is the right thing to do (it may feel like whack-a-mole but isn't)

Eh, it's still whack-a-mole, there just happen to be a finite number of moles :-)