Re: [PATCH 4/4] KVM: TDX: Always honor guest PAT on TDX enabled platforms

[Yan Zhao <yan.y.zhao@intel.com>]

On Mon, Mar 03, 2025 at 05:14:55PM +0100, Paolo Bonzini wrote:
> On 3/3/25 02:30, Yan Zhao wrote:
> > >   	kvm->arch.has_protected_state = true;
> > >   	kvm->arch.has_private_mem = true;
> > > +	kvm->arch.disabled_quirks |= KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT;
> > Though the quirk is disabled by default in KVM in tdx_vm_init() for TDs, the
> > kvm->arch.disabled_quirks can be overwritten by a userspace specified value in
> > kvm_vm_ioctl_enable_cap().
> > "kvm->arch.disabled_quirks = cap->args[0] & kvm_caps.supported_quirks;"
> > 
> > So, when an old userspace tries to disable other quirks on this new KVM, it may
> > accidentally turn KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT into enabled for TDs, which
> > would cause SEPT being zapped when (de)attaching non-coherent devices.
> 
> Yeah, sorry about that - Xiaoyao also pointed it out and I should have
> noticed it---or marked the patches as RFC which they were.
> 
> > Could we force KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT to be disabled for TDs?
> > 
> > e.g.
> > 
> > tdx_vm_init
> >     kvm->arch.always_disabled_quirks |= KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT;
> > 
> > static inline bool kvm_check_has_quirk(struct kvm *kvm, u64 quirk)
> > {
> >          WARN_ON_ONCE(kvm->arch.always_disabled_quirk & kvm_caps.force_enabled_quirks);
> > 
> >          u64 disabled_quirks = kvm->arch.always_disabled_quirk | kvm->arch.disabled_quirks;
> >          return !(disabled_quirks & quirk) |
> >                 (kvm_caps.force_enabled_quirks & quirk);
> > }
> 
> We can change KVM_ENABLE_CAP(KVM_X86_DISABLE_QUIRKS), as well as QUIRKS2, to
> use "|=" instead of "=".
> 
> While this is technically a change in the API, the current implementation is
> just awful and I hope that no one is relying on it! This way, the
I think QEMU is not relying on it.

I considered making this change while writing the quirk SLOT_ZAP_ALL but gave it
up, thinking it might allow users to re-enable a quirk later on.

I'm glad you also see it as a bug:)

> "always_disabled_quirks" are not needed.
> 
> If the "|=" idea doesn't work out I agree that
> kvm->arch.always_disabled_quirk is needed.
> 
> Sending v3 shortly...
Thanks!