[v3,03/57] KVM: x86: Do all post-set CPUID processing during vCPU creation

Commit Message

Sean Christopherson Nov. 28, 2024, 1:33 a.m. UTC

During vCPU creation, process KVM's default, empty CPUID as if userspace
set an empty CPUID to ensure consistent and correct behavior with respect
to guest CPUID.  E.g. if userspace never sets guest CPUID, KVM will never
configure cr4_guest_rsvd_bits, and thus create divergent, incorrect, guest-
visible behavior due to letting the guest set any KVM-supported CR4 bits
despite the features not being allowed per guest CPUID.

Note!  This changes KVM's ABI, as lack of full CPUID processing allowed
userspace to stuff garbage vCPU state, e.g. userspace could set CR4 to a
guest-unsupported value via KVM_SET_SREGS.  But it's extremely unlikely
that this is a breaking change, as KVM already has many flows that require
userspace to set guest CPUID before loading vCPU state.  E.g. multiple MSR
flows consult guest CPUID on host writes, and KVM_SET_SREGS itself already
relies on guest CPUID being up-to-date, as KVM's validity check on CR3
consumes CPUID.0x7.1 (for LAM) and CPUID.0x80000008 (for MAXPHYADDR).

Furthermore, the plan is to commit to enforcing guest CPUID for userspace
writes to MSRs, at which point bypassing sregs CPUID checks is even more
nonsensical.

Signed-off-by: Sean Christopherson <[e-mail]>
---
 arch/x86/kvm/cpuid.c | 2 +-
 arch/x86/kvm/cpuid.h | 1 +
 arch/x86/kvm/x86.c   | 1 +
 3 files changed, 3 insertions(+), 1 deletion(-)

Patch

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index eb4b32bcfa56..b9ad07e24160 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -338,7 +338,7 @@  static bool guest_cpuid_is_amd_or_hygon(struct kvm_vcpu *vcpu)
 	       is_guest_vendor_hygon(entry->ebx, entry->ecx, entry->edx);
 }
 
-static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
+void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 {
 	struct kvm_lapic *apic = vcpu->arch.apic;
 	struct kvm_cpuid_entry2 *best;
diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
index c8dc66eddefd..e51b868e9d36 100644
--- a/arch/x86/kvm/cpuid.h
+++ b/arch/x86/kvm/cpuid.h
@@ -10,6 +10,7 @@ 
 extern u32 kvm_cpu_caps[NR_KVM_CPU_CAPS] __read_mostly;
 void kvm_set_cpu_caps(void);
 
+void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu);
 void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu);
 void kvm_update_pv_runtime(struct kvm_vcpu *vcpu);
 struct kvm_cpuid_entry2 *kvm_find_cpuid_entry_index(struct kvm_vcpu *vcpu,
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2e713480933a..ca9b0a00cbcc 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -12301,6 +12301,7 @@  int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
 
 	kvm_xen_init_vcpu(vcpu);
 	vcpu_load(vcpu);
+	kvm_vcpu_after_set_cpuid(vcpu);
 	kvm_set_tsc_khz(vcpu, vcpu->kvm->arch.default_tsc_khz);
 	kvm_vcpu_reset(vcpu, false);
 	kvm_init_mmu(vcpu);