Re: [PATCH 05/15] KVM: VMX: Add document to state that write to uret msr should always be intercepted

From: Paolo Bonzini
Date: Thu Nov 18 2021 - 11:05:39 EST

On 11/18/21 12:08, Lai Jiangshan wrote:
From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>

And adds a corresponding sanity check code.

Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
arch/x86/kvm/vmx/vmx.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index e8a41fdc3c4d..cd081219b668 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -3703,13 +3703,21 @@ void vmx_disable_intercept_for_msr(struct kvm_vcpu *vcpu, u32 msr, int type)
if (!cpu_has_vmx_msr_bitmap())
+ /*
+ * Write to uret msr should always be intercepted due to the mechanism
+ * must know the current value. Santity check to avoid any inadvertent
+ * mistake in coding.
+ */
+ if (WARN_ON_ONCE(vmx_find_uret_msr(vmx, msr) && (type & MSR_TYPE_W)))
+ return;

I'm not sure about this one, it's relatively expensive to call vmx_find_uret_msr.

User-return MSRs and disable-intercept MSRs are almost the opposite: uret is for MSRs that the host (not even the processor) never uses, disable-intercept is for MSRs that the guest reads/writes often. As such it seems almost impossible that they overlap.