[PATCH v2 3/3] KVM: VMX: Gracefully handle faults on VMXON

[Sean Christopherson]

Gracefully handle faults on VMXON, e.g. #GP due to VMX being disabled by
BIOS, instead of letting the fault crash the system.  Now that KVM uses
cpufeatures to query support instead of reading MSR_IA32_FEAT_CTL
directly, it's possible for a bug in a different subsystem to cause KVM
to incorrectly attempt VMXON[*].  Crashing the system is especially
annoying if the system is configured such that hardware_enable() will
be triggered during boot.

Oppurtunistically rename @addr to @vmxon_pointer and use a named param
to reference it in the inline assembly.

Print 0xdeadbeef in the ultra-"rare" case that reading MSR_IA32_FEAT_CTL
also faults.

[*] https://lkml.kernel.org/r/20200226231615.13664-1-sean.j.christopherson@xxxxxxxxx
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
---
 arch/x86/kvm/vmx/vmx.c | 24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 07634caa560d..3aba51d782e2 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2218,18 +2218,33 @@ static __init int vmx_disabled_by_bios(void)
 	       !boot_cpu_has(X86_FEATURE_VMX);
 }
 
-static void kvm_cpu_vmxon(u64 addr)
+static int kvm_cpu_vmxon(u64 vmxon_pointer)
 {
+	u64 msr;
+
 	cr4_set_bits(X86_CR4_VMXE);
 	intel_pt_handle_vmx(1);
 
-	asm volatile ("vmxon %0" : : "m"(addr));
+	asm_volatile_goto("1: vmxon %[vmxon_pointer]\n\t"
+			  _ASM_EXTABLE(1b, %l[fault])
+			  : : [vmxon_pointer] "m"(vmxon_pointer)
+			  : : fault);
+	return 0;
+
+fault:
+	WARN_ONCE(1, "VMXON faulted, MSR_IA32_FEAT_CTL (0x3a) = 0x%llx\n",
+		  rdmsrl_safe(MSR_IA32_FEAT_CTL, &msr) ? 0xdeadbeef : msr);
+	intel_pt_handle_vmx(0);
+	cr4_clear_bits(X86_CR4_VMXE);
+
+	return -EFAULT;
 }
 
 static int hardware_enable(void)
 {
 	int cpu = raw_smp_processor_id();
 	u64 phys_addr = __pa(per_cpu(vmxarea, cpu));
+	int r;
 
 	if (cr4_read_shadow() & X86_CR4_VMXE)
 		return -EBUSY;
@@ -2246,7 +2261,10 @@ static int hardware_enable(void)
 	INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
 	spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
 
-	kvm_cpu_vmxon(phys_addr);
+	r = kvm_cpu_vmxon(phys_addr);
+	if (r)
+		return r;
+
 	if (enable_ept)
 		ept_sync_global();
 
-- 
2.24.1