[3/3] KVM: TDX: Always honor guest PAT on TDX enabled platforms

Commit Message

Yan Zhao Feb. 24, 2025, 7:10 a.m. UTC

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,
which would cause mirrored EPTs for TDs to be zapped, leading to the
TDX-module-managed private EPT being incorrectly zapped.

As a new platform, TDX is always with self-snoop feature supported and has
no worry to break old not-well-written yet unmodifiable guests. So, simply
make the quirk KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT invalid on TDX enabled
platforms.

Suggested-by: Sean Christopherson <[e-mail]>
Signed-off-by: Yan Zhao <[e-mail]>
---
 Documentation/virt/kvm/api.rst | 20 +++++++++++---------
 arch/x86/kvm/vmx/main.c        |  1 +
 arch/x86/kvm/vmx/tdx.c         |  5 +++++
 3 files changed, 17 insertions(+), 9 deletions(-)

Patch

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index c22211c2f54c..5954c5cde33d 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -8165,9 +8165,11 @@  KVM_X86_QUIRK_STUFF_FEATURE_MSRS    By default, at vCPU creation, KVM sets the
                                     be set by userspace (KVM sets them based on
                                     guest CPUID, for safety purposes).
 
-KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT  By default, on Intel platforms, KVM ignores
-                                    guest PAT and forces the effective memory
-                                    type to WB in EPT.  The quirk has no effect
+KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT  By default, on Intel platforms except TDX,
+                                    KVM ignores guest PAT and forces the
+                                    effective memory type to WB in EPT. The
+                                    quirk only affects the memory type of
+                                    KVM-managed EPTs.  The quirk has no effect
                                     when KVM runs on Intel platforms which are
                                     incapable of safely honoring guest PAT
                                     (i.e., without CPU feature self-snoop, KVM
@@ -8184,14 +8186,14 @@  KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT  By default, on Intel platforms, KVM ignores
                                     map the video RAM, causing wayland desktop
                                     to fail to start correctly). To prevent
                                     breaking older guest software, KVM enables
-                                    the quirk by default on Intel platforms.
-                                    Userspace can disable the quirk to honor
-                                    guest PAT when there is no older
+                                    the quirk by default on Intel platforms
+                                    except TDX. Userspace can disable the quirk
+                                    to honor guest PAT when there is no older
                                     unmodifiable guest software that relies on
                                     KVM to force memory type to WB.  Note, the
-                                    quirk is not visible on AMD's platforms,
-                                    i.e., KVM always honors guest PAT when
-                                    running on AMD.
+                                    quirk is not visible on Intel TDX or AMD's
+                                    platforms, i.e., KVM always honors guest PAT
+                                    when running on Intel TDX or AMD.
 =================================== ============================================
 
 7.32 KVM_CAP_MAX_VCPU_ID
diff --git a/arch/x86/kvm/vmx/main.c b/arch/x86/kvm/vmx/main.c
index f586e09b5acf..1fa0364faa60 100644
--- a/arch/x86/kvm/vmx/main.c
+++ b/arch/x86/kvm/vmx/main.c
@@ -1092,6 +1092,7 @@  static int __init vt_init(void)
 		vcpu_align = max_t(unsigned, vcpu_align,
 				__alignof__(struct vcpu_tdx));
 		kvm_caps.supported_vm_types |= BIT(KVM_X86_TDX_VM);
+		kvm_caps.supported_quirks &= ~KVM_X86_QUIRK_EPT_IGNORE_GUEST_PAT;
 	}
 
 	/*
diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index e73c9fcf213c..7d063cacc9c9 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -3483,6 +3483,11 @@  int __init tdx_bringup(void)
 		goto success_disable_tdx;
 	}
 
+	if (!cpu_feature_enabled(X86_FEATURE_SELFSNOOP)) {
+		pr_err("Self-snoop is reqiured for TDX\n");
+		goto success_disable_tdx;
+	}
+
 	if (!kvm_can_support_tdx()) {
 		pr_err("tdx: no TDX private KeyIDs available\n");
 		goto success_disable_tdx;