linux-kernel - [PATCH v2 27/49] KVM: x86: Swap incoming guest CPUID into vCPU before massaging in KVM_SET

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Date: Fri, 17 May 2024 10:39:04 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>, 
	Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Hou Wenlong <houwenlong.hwl@...group.com>, Kechen Lu <kechenl@...dia.com>, 
	Oliver Upton <oliver.upton@...ux.dev>, Maxim Levitsky <mlevitsk@...hat.com>, 
	Binbin Wu <binbin.wu@...ux.intel.com>, Yang Weijiang <weijiang.yang@...el.com>, 
	Robert Hoo <robert.hoo.linux@...il.com>
Subject: [PATCH v2 27/49] KVM: x86: Swap incoming guest CPUID into vCPU before
 massaging in KVM_SET_CPUID2

When handling KVM_SET_CPUID{,2}, swap the old and new CPUID arrays and
lengths before processing the new CPUID, and simply undo the swap if
setting the new CPUID fails for whatever reason.

To keep the diff reasonable, continue passing the entry array and length
to most helpers, and defer the more complete cleanup to future commits.

For any sane VMM, setting "bad" CPUID state is not a hot path (or even
something that is surviable), and setting guest CPUID before it's known
good will allow removing all of KVM's infrastructure for processing CPUID
entries directly (as opposed to operating on vcpu->arch.cpuid_entries).

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/cpuid.c | 49 +++++++++++++++++++++++++++-----------------
 1 file changed, 30 insertions(+), 19 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 33e3e77de1b7..4ad01867cb8d 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -175,10 +175,10 @@ static inline struct kvm_cpuid_entry2 *cpuid_entry2_find(
 	return NULL;
 }
 
-static int kvm_check_cpuid(struct kvm_vcpu *vcpu,
-			   struct kvm_cpuid_entry2 *entries,
-			   int nent)
+static int kvm_check_cpuid(struct kvm_vcpu *vcpu)
 {
+	struct kvm_cpuid_entry2 *entries = vcpu->arch.cpuid_entries;
+	int nent = vcpu->arch.cpuid_nent;
 	struct kvm_cpuid_entry2 *best;
 	u64 xfeatures;
 
@@ -369,9 +369,11 @@ void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu)
 }
 EXPORT_SYMBOL_GPL(kvm_update_cpuid_runtime);
 
-static bool kvm_cpuid_has_hyperv(struct kvm_cpuid_entry2 *entries, int nent)
+static bool kvm_cpuid_has_hyperv(struct kvm_vcpu *vcpu)
 {
 #ifdef CONFIG_KVM_HYPERV
+	struct kvm_cpuid_entry2 *entries = vcpu->arch.cpuid_entries;
+	int nent = vcpu->arch.cpuid_nent;
 	struct kvm_cpuid_entry2 *entry;
 
 	entry = cpuid_entry2_find(entries, nent, HYPERV_CPUID_INTERFACE,
@@ -436,8 +438,7 @@ void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 					 __cr4_reserved_bits(guest_cpuid_has, vcpu);
 #undef __kvm_cpu_cap_has
 
-	kvm_hv_set_cpuid(vcpu, kvm_cpuid_has_hyperv(vcpu->arch.cpuid_entries,
-						    vcpu->arch.cpuid_nent));
+	kvm_hv_set_cpuid(vcpu, kvm_cpuid_has_hyperv(vcpu));
 
 	/* Invoke the vendor callback only after the above state is updated. */
 	static_call(kvm_x86_vcpu_after_set_cpuid)(vcpu);
@@ -478,6 +479,15 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
 {
 	int r;
 
+	/*
+	 * Swap the existing (old) entries with the incoming (new) entries in
+	 * order to massage the new entries, e.g. to account for dynamic bits
+	 * that KVM controls, without clobbering the current guest CPUID, which
+	 * KVM needs to preserve in order to unwind on failure.
+	 */
+	swap(vcpu->arch.cpuid_entries, e2);
+	swap(vcpu->arch.cpuid_nent, nent);
+
 	/*
 	 * KVM does not correctly handle changing guest CPUID after KVM_RUN, as
 	 * MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, etc.. aren't
@@ -497,31 +507,25 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
 		 * only because any change in CPUID is disallowed, i.e. using
 		 * stale data is ok because KVM will reject the change.
 		 */
-		__kvm_update_cpuid_runtime(vcpu, e2, nent);
+		kvm_update_cpuid_runtime(vcpu);
 
 		r = kvm_cpuid_check_equal(vcpu, e2, nent);
 		if (r)
-			return r;
-
-		kvfree(e2);
-		return 0;
+			goto err;
+		goto success;
 	}
 
 #ifdef CONFIG_KVM_HYPERV
-	if (kvm_cpuid_has_hyperv(e2, nent)) {
+	if (kvm_cpuid_has_hyperv(vcpu)) {
 		r = kvm_hv_vcpu_init(vcpu);
 		if (r)
-			return r;
+			goto err;
 	}
 #endif
 
-	r = kvm_check_cpuid(vcpu, e2, nent);
+	r = kvm_check_cpuid(vcpu);
 	if (r)
-		return r;
-
-	kvfree(vcpu->arch.cpuid_entries);
-	vcpu->arch.cpuid_entries = e2;
-	vcpu->arch.cpuid_nent = nent;
+		goto err;
 
 	vcpu->arch.kvm_cpuid = kvm_get_hypervisor_cpuid(vcpu, KVM_SIGNATURE);
 #ifdef CONFIG_KVM_XEN
@@ -529,7 +533,14 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
 #endif
 	kvm_vcpu_after_set_cpuid(vcpu);
 
+success:
+	kvfree(e2);
 	return 0;
+
+err:
+	swap(vcpu->arch.cpuid_entries, e2);
+	swap(vcpu->arch.cpuid_nent, nent);
+	return r;
 }
 
 /* when an old userspace process fills a new kernel module */
-- 
2.45.0.215.g3402c0e53f-goog