On Sat, Mar 01, 2025 at 02:34:28AM -0500, Paolo Bonzini wrote:
> From: Yan Zhao <yan.y.zhao@xxxxxxxxx>
>
> Always honor guest PAT in KVM-managed EPTs on TDX enabled platforms by
> making self-snoop feature a hard dependency for TDX and making quirk
> KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT not a valid quirk once TDX is enabled.
>
> The quirk KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT only affects memory type of
> KVM-managed EPTs. For the TDX-module-managed private EPT, memory type is
> always forced to WB now.
>
> Honoring guest PAT in KVM-managed EPTs ensures KVM does not invoke
> kvm_zap_gfn_range() when attaching/detaching non-coherent DMA devices;
> this would cause mirrored EPTs for TDs to be zapped, as well as incorrect
> zapping of the private EPT that is managed by the TDX module.
>
> As a new platform, TDX always comes with self-snoop feature supported and has
> no worry to break old not-well-written yet unmodifiable guests. So, simply
> force-disable the KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT quirk for TDX VMs.
>
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> Message-ID: <20250224071039.31511-1-yan.y.zhao@xxxxxxxxx>
> [Use disabled_quirks instead of supported_quirks. - Paolo]
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
> arch/x86/kvm/vmx/tdx.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index b6f6f6e2f02e..4450fd99cb4c 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -624,6 +624,7 @@ int tdx_vm_init(struct kvm *kvm)
>
> 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.
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);
}
>
> /*
> * Because guest TD is protected, VMM can't parse the instruction in TD.
> @@ -3470,6 +3471,11 @@ int __init tdx_bringup(void)
> goto success_disable_tdx;
> }
>
> + if (!cpu_feature_enabled(X86_FEATURE_SELFSNOOP)) {
> + pr_err("Self-snoop is required for TDX\n");
> + goto success_disable_tdx;
> + }
> +
> if (!cpu_feature_enabled(X86_FEATURE_TDX_HOST_PLATFORM)) {
> pr_err("tdx: no TDX private KeyIDs available\n");
> goto success_disable_tdx;
> --
> 2.43.5
>
