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Message-ID: <aIEkrc7Mdf2ia1Mm@google.com>
Date: Wed, 23 Jul 2025 11:06:37 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Zack Rusin <zack.rusin@...adcom.com>
Cc: linux-kernel@...r.kernel.org, Doug Covelli <doug.covelli@...adcom.com>,
Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>,
Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
"H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v2 2/5] KVM: x86: Allow enabling of the vmware backdoor
via a cap
On Tue, Apr 22, 2025, Zack Rusin wrote:
> @@ -6735,6 +6734,19 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
> mutex_unlock(&kvm->lock);
> break;
> }
> +#ifdef CONFIG_KVM_VMWARE
> + case KVM_CAP_X86_VMWARE_BACKDOOR:
I much perfer using a single KVM_CAP_X86_VMWARE capability. More below.
> + r = -EINVAL;
> + if (cap->args[0] & ~1)
Using bit 0 for "enable" needs to be #defined arch/x86/include/uapi/asm/kvm.h.
At that point, adding more capabilities for the other VMware functionality doesn't
make much sense, especially since the capabilities that are added in later patches
don't have the kvm->created_vcpus protection, i.e. are likely buggy.
E.g. with the below diff (completely untested, probably won't apply cleanly?)
spread across three-ish patches, the accessors can be:
static inline bool kvm_is_vmware_cap_enabled(struct kvm *kvm, u64 cap)
{
return kvm->arch.vmware.caps & cap;
}
static inline bool kvm_is_vmware_backdoor_enabled(struct kvm_vcpu *vcpu)
{
return kvm_is_vmware_cap_enabled(kvm, KVM_VMWARE_ENABLE_BACKDOOR);
}
static inline bool kvm_is_vmware_hypercall_enabled(struct kvm *kvm)
{
return kvm_is_vmware_cap_enabled(kvm, KVM_VMWARE_ENABLE_HYPERCALL);
}
static inline bool kvm_vmware_nested_backdoor_l0_enabled(struct kvm *kvm)
{
return kvm_is_vmware_backdoor_enabled(kvm) &&
kvm_is_vmware_cap_enabled(kvm, KVM_VMWARE_ENABLE_NESTED_BACKDOOR);
}
---
arch/x86/include/asm/kvm_host.h | 4 +---
arch/x86/include/uapi/asm/kvm.h | 4 ++++
arch/x86/kvm/x86.c | 34 ++++++++++++---------------------
3 files changed, 17 insertions(+), 25 deletions(-)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 639c49db0106..1433cdd14675 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1219,9 +1219,7 @@ struct kvm_xen {
#ifdef CONFIG_KVM_VMWARE
/* VMware emulation context */
struct kvm_vmware {
- bool backdoor_enabled;
- bool hypercall_enabled;
- bool nested_backdoor_l0_enabled;
+ u64 caps;
};
#endif
diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index e019111e2150..ae578422d6f4 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -1013,4 +1013,8 @@ struct kvm_tdx_init_mem_region {
__u64 nr_pages;
};
+#define KVM_VMWARE_ENABLE_BACKDOOR _BITULL(0)
+#define KVM_VMWARE_ENABLE_HYPERCALL _BITULL(1)
+#define KVM_VMWARE_ENABLE_NESTED_BACKDOOR _BITULL(2)
+
#endif /* _ASM_X86_KVM_H */
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7234333a92d8..b9e2faf0ceb7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -126,6 +126,10 @@ static u64 __read_mostly efer_reserved_bits = ~((u64)EFER_SCE);
#define KVM_X2APIC_API_VALID_FLAGS (KVM_X2APIC_API_USE_32BIT_IDS | \
KVM_X2APIC_API_DISABLE_BROADCAST_QUIRK)
+#define KVM_CAP_VMWARE_VALID_MASK (KVM_VMWARE_CAP_ENABLE_BACKDOOR | \
+ KVM_VMWARE_ENABLE_HYPERCALL | \
+ KVM_VMWARE_ENABLE_NESTED_BACKDOOR)
+
static void update_cr8_intercept(struct kvm_vcpu *vcpu);
static void process_nmi(struct kvm_vcpu *vcpu);
static void __kvm_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags);
@@ -4708,11 +4712,6 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
case KVM_CAP_IRQFD_RESAMPLE:
case KVM_CAP_MEMORY_FAULT_INFO:
case KVM_CAP_X86_GUEST_MODE:
-#ifdef CONFIG_KVM_VMWARE
- case KVM_CAP_X86_VMWARE_BACKDOOR:
- case KVM_CAP_X86_VMWARE_HYPERCALL:
- case KVM_CAP_X86_VMWARE_NESTED_BACKDOOR_L0:
-#endif
r = 1;
break;
case KVM_CAP_PRE_FAULT_MEMORY:
@@ -4836,6 +4835,10 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
case KVM_CAP_READONLY_MEM:
r = kvm ? kvm_arch_has_readonly_mem(kvm) : 1;
break;
+#ifdef CONFIG_KVM_VMWARE
+ case KVM_CAP_X86_VMWARE:
+ return KVM_CAP_VMWARE_VALID_MASK;
+#endif
default:
break;
}
@@ -6669,31 +6672,18 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
break;
}
#ifdef CONFIG_KVM_VMWARE
- case KVM_CAP_X86_VMWARE_BACKDOOR:
+ case KVM_CAP_X86_VMWARE:
r = -EINVAL;
- if (cap->args[0] & ~1)
+ if (cap->args[0] & ~KVM_CAP_VMWARE_VALID_MASK)
break;
+
mutex_lock(&kvm->lock);
if (!kvm->created_vcpus) {
- kvm->arch.vmware.backdoor_enabled = cap->args[0];
+ kvm->arch.vmware.caps = cap->args[0];
r = 0;
}
mutex_unlock(&kvm->lock);
break;
- case KVM_CAP_X86_VMWARE_HYPERCALL:
- r = -EINVAL;
- if (cap->args[0] & ~1)
- break;
- kvm->arch.vmware.hypercall_enabled = cap->args[0];
- r = 0;
- break;
- case KVM_CAP_X86_VMWARE_NESTED_BACKDOOR_L0:
- r = -EINVAL;
- if (cap->args[0] & ~1)
- break;
- kvm->arch.vmware.nested_backdoor_l0_enabled = cap->args[0];
- r = 0;
- break;
#endif
default:
r = -EINVAL;
base-commit: 77a53b6f5d1c2dabef34d890d212910ed1f43bcb
--
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