Re: [RFC PATCH 3/3] KVM: x86: Disallow writes to immutable feature MSRs after KVM_RUN

From: Sean Christopherson <seanjc@google.com>
To: Xiaoyao Li <xiaoyao.li@intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Like Xu <like.xu.linux@gmail.com>
Subject: Re: [RFC PATCH 3/3] KVM: x86: Disallow writes to immutable feature MSRs after KVM_RUN
Date: Wed, 10 Aug 2022 14:45:39 +0000	[thread overview]
Message-ID: <YvPEk4tnvajOfjBl@google.com> (raw)
In-Reply-To: <40c9ecc1-e223-160b-4939-07e4f7200781@intel.com>

On Wed, Aug 10, 2022, Xiaoyao Li wrote:
> On 8/6/2022 1:29 AM, Sean Christopherson wrote:
> > @@ -2136,6 +2156,23 @@ static int do_get_msr(struct kvm_vcpu *vcpu, unsigned index, u64 *data)
> >   static int do_set_msr(struct kvm_vcpu *vcpu, unsigned index, u64 *data)
> >   {
> > +	u64 val;
> > +
> > +	/*
> > +	 * Disallow writes to immutable feature MSRs after KVM_RUN.  KVM does
> > +	 * not support modifying the guest vCPU model on the fly, e.g. changing
> > +	 * the nVMX capabilities while L2 is running is nonsensical.  Ignore
> > +	 * writes of the same value, e.g. to allow userspace to blindly stuff
> > +	 * all MSRs when emulating RESET.
> > +	 */
> > +	if (vcpu->arch.last_vmentry_cpu != -1 &&
> 
> can we extract "vcpu->arch.last_vmentry_cpu != -1" into a function like
> kvm_vcpu_has_runned() ?

Ya, a helper is in order.  I'll add a patch in the next version.