KVM: x86: Allow vendor code to disable quirks

In some cases, the handling of quirks is split between platform-specific
code and generic code, or it is done entirely in generic code, but the
relevant bug does not trigger on some platforms; for example,
this will be the case for "ignore guest PAT".  Allow unaffected vendor
modules to disable handling of a quirk for all VMs via a new entry in
kvm_caps.

Such quirks remain available in KVM_CAP_DISABLE_QUIRKS2, because that API
tells userspace that KVM *knows* that some of its past behavior was bogus
or just undesirable.  In other words, it's plausible for userspace to
refuse to run if a quirk is not listed by KVM_CAP_DISABLE_QUIRKS2, so
preserve that and make it part of the API.

As an example, mark KVM_X86_QUIRK_CD_NW_CLEARED as auto-disabled on
Intel systems.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

4 files changed, 7 insertions, 0 deletions

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 7fdbe84374f46..bf1dc79fb0c6b 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -2422,6 +2422,9 @@ int memslot_rmap_alloc(struct kvm_memory_slot *slot, unsigned long npages);
 	 KVM_X86_QUIRK_SLOT_ZAP_ALL |		\
 	 KVM_X86_QUIRK_STUFF_FEATURE_MSRS)
 
+#define KVM_X86_CONDITIONAL_QUIRKS		\
+	 KVM_X86_QUIRK_CD_NW_CLEARED
+
 /*
  * KVM previously used a u32 field in kvm_run to indicate the hypercall was
  * initiated from long mode. KVM now sets bit 0 to indicate long mode, but the
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index e67de787fc714..92c1b0fffe6b9 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5472,6 +5472,7 @@ static __init int svm_hardware_setup(void)
 	 */
 	allow_smaller_maxphyaddr = !npt_enabled;
 
+	kvm_caps.inapplicable_quirks &= ~KVM_X86_QUIRK_CD_NW_CLEARED;
 	return 0;
 
 err:
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index cdc9c7b965114..93168a46adcf7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9783,6 +9783,7 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
 		kvm_host.xcr0 = xgetbv(XCR_XFEATURE_ENABLED_MASK);
 		kvm_caps.supported_xcr0 = kvm_host.xcr0 & KVM_SUPPORTED_XCR0;
 	}
+	kvm_caps.inapplicable_quirks = KVM_X86_CONDITIONAL_QUIRKS;
 
 	rdmsrl_safe(MSR_EFER, &kvm_host.efer);
 
@@ -12733,6 +12734,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	/* Decided by the vendor code for other VM types.  */
 	kvm->arch.pre_fault_allowed =
 		type == KVM_X86_DEFAULT_VM || type == KVM_X86_SW_PROTECTED_VM;
+	kvm->arch.disabled_quirks = kvm_caps.inapplicable_quirks;
 
 	ret = kvm_page_track_init(kvm);
 	if (ret)
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 8ce6da98b5a23..221778792c3ca 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -34,6 +34,7 @@ struct kvm_caps {
 	u64 supported_xcr0;
 	u64 supported_xss;
 	u64 supported_perf_cap;
+	u64 inapplicable_quirks;
 };
 
 struct kvm_host_values {