linux-kernel - [PATCH 4/4] KVM: TDX: Always honor guest PAT on TDX enabled platforms

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Message-ID: <20250301073428.2435768-5-pbonzini@redhat.com>
Date: Sat,  1 Mar 2025 02:34:28 -0500
From: Paolo Bonzini <pbonzini@...hat.com>
To: linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc: seanjc@...gle.com,
	yan.y.zhao@...el.com
Subject: [PATCH 4/4] KVM: TDX: Always honor guest PAT on TDX enabled platforms

From: Yan Zhao <yan.y.zhao@...el.com>

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@...gle.com>
Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
Message-ID: <20250224071039.31511-1-yan.y.zhao@...el.com>
[Use disabled_quirks instead of supported_quirks. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 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;

 	/*
 	 * 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