Re: [PATCH] KVM: VMX: Do not trap VMFUNC instructions for L1 guests.

[Sean Christopherson <seanjc@google.com>]

On Mon, Nov 07, 2022, Paolo Bonzini wrote:
> On 11/7/22 09:27, Yu Zhang wrote:
> > VMFUNC is not supported for L1 guests, and executing VMFUNC in
> > L1 shall generate a #UD directly. Just disable it in secondary
> > proc-based execution control for L1, instead of intercepting it
> > and inject the #UD again.
> > 
> > Signed-off-by: Yu Zhang<yu.c.zhang@linux.intel.com>
> 
> Is this for TDX or similar?  The reason for a patch should be mentioned in
> the commit message.

It's just a cleanup, but (a) it should be split over two patches as disabling
VMFUNC for L1 is technically a functional change, where as the changes to
nested_vmx_setup_ctls_msrs() are pure cleanups, and (b) the !guest_mode path in
handle_vmfunc() should either be removed or turned into a KVM_BUG_ON().

E.g.

---
 arch/x86/kvm/vmx/nested.c | 11 ++---------
 arch/x86/kvm/vmx/vmx.c    |  7 ++++++-
 2 files changed, 8 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 0c62352dda6a..fa4130361187 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -5792,15 +5792,8 @@ static int handle_vmfunc(struct kvm_vcpu *vcpu)
 	struct vmcs12 *vmcs12;
 	u32 function = kvm_rax_read(vcpu);
 
-	/*
-	 * VMFUNC is only supported for nested guests, but we always enable the
-	 * secondary control for simplicity; for non-nested mode, fake that we
-	 * didn't by injecting #UD.
-	 */
-	if (!is_guest_mode(vcpu)) {
-		kvm_queue_exception(vcpu, UD_VECTOR);
-		return 1;
-	}
+	if (KVM_BUG_ON(!is_guest_mode(vcpu), vcpu->kvm))
+		return -EIO;
 
 	vmcs12 = get_vmcs12(vcpu);
 
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 63247c57c72c..5a66c3c16c2d 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4487,6 +4487,12 @@ static u32 vmx_secondary_exec_control(struct vcpu_vmx *vmx)
 				  SECONDARY_EXEC_VIRTUAL_INTR_DELIVERY);
 	exec_control &= ~SECONDARY_EXEC_VIRTUALIZE_X2APIC_MODE;
 
+	/*
+	 * KVM doesn't support VMFUNC for L1, but the control is set in KVM's
+	 * base configuration as KVM emulates VMFUNC[EPTP_SWITCHING] for L2.
+	 */
+	exec_control &= ~SECONDARY_EXEC_ENABLE_VMFUNC;
+
 	/* SECONDARY_EXEC_DESC is enabled/disabled on writes to CR4.UMIP,
 	 * in vmx_set_cr4.  */
 	exec_control &= ~SECONDARY_EXEC_DESC;
@@ -6004,7 +6010,6 @@ static int (*kvm_vmx_exit_handlers[])(struct kvm_vcpu *vcpu) = {
 	[EXIT_REASON_RDSEED]                  = kvm_handle_invalid_op,
 	[EXIT_REASON_PML_FULL]		      = handle_pml_full,
 	[EXIT_REASON_INVPCID]                 = handle_invpcid,
-	[EXIT_REASON_VMFUNC]		      = handle_vmx_instruction,
 	[EXIT_REASON_PREEMPTION_TIMER]	      = handle_preemption_timer,
 	[EXIT_REASON_ENCLS]		      = handle_encls,
 	[EXIT_REASON_BUS_LOCK]                = handle_bus_lock_vmexit,

base-commit: 07341b10fcbd5a7ef18225e0e9a8a40d91e3a2cc
-- 


and then the pure cleanup that is made possible because KVM now does:

	msrs->secondary_ctls_high = vmcs_conf->cpu_based_2nd_exec_ctrl;

---
 arch/x86/kvm/vmx/nested.c | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index fa4130361187..981bf5b3a319 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -6801,6 +6801,7 @@ void nested_vmx_setup_ctls_msrs(struct vmcs_config *vmcs_conf, u32 ept_caps)
 		SECONDARY_EXEC_VIRTUAL_INTR_DELIVERY |
 		SECONDARY_EXEC_RDRAND_EXITING |
 		SECONDARY_EXEC_ENABLE_INVPCID |
+		SECONDARY_EXEC_ENABLE_VMFUNC |
 		SECONDARY_EXEC_RDSEED_EXITING |
 		SECONDARY_EXEC_XSAVES |
 		SECONDARY_EXEC_TSC_SCALING;
@@ -6832,18 +6833,13 @@ void nested_vmx_setup_ctls_msrs(struct vmcs_config *vmcs_conf, u32 ept_caps)
 				SECONDARY_EXEC_ENABLE_PML;
 			msrs->ept_caps |= VMX_EPT_AD_BIT;
 		}
-	}
 
-	if (cpu_has_vmx_vmfunc()) {
-		msrs->secondary_ctls_high |=
-			SECONDARY_EXEC_ENABLE_VMFUNC;
 		/*
-		 * Advertise EPTP switching unconditionally
-		 * since we emulate it
+		 * Advertise EPTP switching irrespective of hardware support,
+		 * KVM emulates it in software so long as VMFUNC is supported.
 		 */
-		if (enable_ept)
-			msrs->vmfunc_controls =
-				VMX_VMFUNC_EPTP_SWITCHING;
+		if (cpu_has_vmx_vmfunc())
+			msrs->vmfunc_controls = VMX_VMFUNC_EPTP_SWITCHING;
 	}
 
 	/*

base-commit: 777dde94dd5e4328b419dcc5cb7118b39588eab1
--