From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>,
Igor Mammedov <imammedo@redhat.com>,
linux-kernel@vger.kernel.org
Subject: [PATCH v2 2/4] KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM_RUN
Date: Mon, 17 Jan 2022 16:05:40 +0100 [thread overview]
Message-ID: <20220117150542.2176196-3-vkuznets@redhat.com> (raw)
In-Reply-To: <20220117150542.2176196-1-vkuznets@redhat.com>
Commit feb627e8d6f6 ("KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN")
forbade changing CPUID altogether but unfortunately this is not fully
compatible with existing VMMs. In particular, QEMU reuses vCPU fds for
CPU hotplug after unplug and it calls KVM_SET_CPUID2. Instead of full ban,
check whether the supplied CPUID data is equal to what was previously set.
Reported-by: Igor Mammedov <imammedo@redhat.com>
Fixes: feb627e8d6f6 ("KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN")
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
arch/x86/kvm/cpuid.c | 33 +++++++++++++++++++++++++++++++++
arch/x86/kvm/x86.c | 19 -------------------
2 files changed, 33 insertions(+), 19 deletions(-)
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 812190a707f6..c2e9c860fc3f 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -119,6 +119,25 @@ static int kvm_check_cpuid(struct kvm_vcpu *vcpu,
return fpu_enable_guest_xfd_features(&vcpu->arch.guest_fpu, xfeatures);
}
+/* Check whether the supplied CPUID data is equal to what is already set for the vCPU. */
+static int kvm_cpuid_check_equal(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
+ int nent)
+{
+ struct kvm_cpuid_entry2 *best;
+ int i;
+
+ for (i = 0; i < nent; i++) {
+ best = kvm_find_cpuid_entry(vcpu, e2[i].function, e2[i].index);
+ if (!best)
+ return -EINVAL;
+
+ if (e2[i].eax != best->eax || e2[i].ebx != best->ebx ||
+ e2[i].ecx != best->ecx || e2[i].edx != best->edx)
+ return -EINVAL;
+ }
+
+ return 0;
+}
static void kvm_update_kvm_cpuid_base(struct kvm_vcpu *vcpu)
{
u32 function;
@@ -313,6 +332,20 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
__kvm_update_cpuid_runtime(vcpu, e2, nent);
+ /*
+ * KVM does not correctly handle changing guest CPUID after KVM_RUN, as
+ * MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, etc.. aren't
+ * tracked in kvm_mmu_page_role. As a result, KVM may miss guest page
+ * faults due to reusing SPs/SPTEs. In practice no sane VMM mucks with
+ * the core vCPU model on the fly. It would've been better to forbid any
+ * KVM_SET_CPUID{,2} calls after KVM_RUN altogether but unfortunately
+ * some VMMs (e.g. QEMU) reuse vCPU fds for CPU hotplug/unplug and do
+ * KVM_SET_CPUID{,2} again. To support this legacy behavior, check
+ * whether the supplied CPUID data is equal to what's already set.
+ */
+ if (vcpu->arch.last_vmentry_cpu != -1)
+ return kvm_cpuid_check_equal(vcpu, e2, nent);
+
r = kvm_check_cpuid(vcpu, e2, nent);
if (r)
return r;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 76b4803dd3bd..ff1416010728 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5230,17 +5230,6 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
struct kvm_cpuid __user *cpuid_arg = argp;
struct kvm_cpuid cpuid;
- /*
- * KVM does not correctly handle changing guest CPUID after KVM_RUN, as
- * MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, etc.. aren't
- * tracked in kvm_mmu_page_role. As a result, KVM may miss guest page
- * faults due to reusing SPs/SPTEs. In practice no sane VMM mucks with
- * the core vCPU model on the fly, so fail.
- */
- r = -EINVAL;
- if (vcpu->arch.last_vmentry_cpu != -1)
- goto out;
-
r = -EFAULT;
if (copy_from_user(&cpuid, cpuid_arg, sizeof(cpuid)))
goto out;
@@ -5251,14 +5240,6 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
struct kvm_cpuid2 __user *cpuid_arg = argp;
struct kvm_cpuid2 cpuid;
- /*
- * KVM_SET_CPUID{,2} after KVM_RUN is forbidded, see the comment in
- * KVM_SET_CPUID case above.
- */
- r = -EINVAL;
- if (vcpu->arch.last_vmentry_cpu != -1)
- goto out;
-
r = -EFAULT;
if (copy_from_user(&cpuid, cpuid_arg, sizeof(cpuid)))
goto out;
--
2.34.1
next prev parent reply other threads:[~2022-01-17 15:05 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-17 15:05 [PATCH v2 0/4] KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM_RUN for CPU hotplug Vitaly Kuznetsov
2022-01-17 15:05 ` [PATCH v2 1/4] KVM: x86: Do runtime CPUID update before updating vcpu->arch.cpuid_entries Vitaly Kuznetsov
2022-01-17 15:05 ` Vitaly Kuznetsov [this message]
2022-01-17 17:30 ` [PATCH v2 2/4] KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM_RUN Paolo Bonzini
2022-01-18 8:40 ` Vitaly Kuznetsov
2022-01-18 14:37 ` Vitaly Kuznetsov
2022-01-17 15:05 ` [PATCH v2 3/4] KVM: selftests: Rename 'get_cpuid_test' to 'cpuid_test' Vitaly Kuznetsov
2022-01-17 15:05 ` [PATCH v2 4/4] KVM: selftests: Test KVM_SET_CPUID2 after KVM_RUN Vitaly Kuznetsov
2022-01-18 14:35 ` [PATCH v2 0/4] KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM_RUN for CPU hotplug Igor Mammedov
2022-01-18 16:17 ` Paolo Bonzini
2022-01-18 16:53 ` Sean Christopherson
2022-01-18 17:12 ` Paolo Bonzini
2022-01-18 16:34 ` Vitaly Kuznetsov
2022-01-19 7:59 ` Igor Mammedov
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