[PATCH v3 2/6] KVM: x86: Allow vendor code to disable quirks

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

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@xxxxxxxxxx>
---
 arch/x86/include/asm/kvm_host.h | 3 +++
 arch/x86/kvm/svm/svm.c          | 1 +
 arch/x86/kvm/x86.c              | 2 ++
 arch/x86/kvm/x86.h              | 1 +
 4 files changed, 7 insertions(+)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 7ebbedc566ff..a4f213d235dd 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -2420,6 +2420,9 @@ 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 ebaa5a41db07..51cfef44b58d 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5426,6 +5426,7 @@ 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 35d03fcdb8e9..5abea6c73a38 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9775,6 +9775,7 @@ 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);
 
@@ -12754,6 +12755,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
 	kvm->arch.apic_bus_cycle_ns = APIC_BUS_CYCLE_NS_DEFAULT;
 	kvm->arch.guest_can_read_msr_platform_info = true;
 	kvm->arch.enable_pmu = enable_pmu;
+	kvm->arch.disabled_quirks = kvm_caps.inapplicable_quirks;
 
 #if IS_ENABLED(CONFIG_HYPERV)
 	spin_lock_init(&kvm->arch.hv_root_tdp_lock);
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 8ce6da98b5a2..221778792c3c 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 {
-- 
2.43.5