Date:   Tue, 7 Dec 2021 20:38:07 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Lai Jiangshan <jiangshanlai@gmail.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Lai Jiangshan <laijs@linux.alibaba.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH 05/15] KVM: VMX: Add document to state that write to uret
 msr should always be intercepted
Message-ID: <Ya/GL0zyobfM1rUF@google.com>
References: <20211118110814.2568-1-jiangshanlai@gmail.com>
 <20211118110814.2568-6-jiangshanlai@gmail.com>
 <226fc242-ae46-3214-4e01-dbfdf5f7c0fb@redhat.com>
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On Thu, Nov 18, 2021, Paolo Bonzini wrote:
> 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.

And they aren't fundamentally mutually exclusive, e.g. KVM could pass-through an
MSR and then do RDMSR in vmx_prepare_switch_to_host() to refresh the uret data
with the current (guest) value.  It'd be silly, but it would work.