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

Paolo Bonzini
2021-11-18T16:05:36Z

On 11/18/21 12:08, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@linux.alibaba.com>
>
> And adds a corresponding sanity check code.
>
> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> ---
> 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())
> return;
>
> + /*
> + * 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.

Paolo