Re: [PATCH v6 12/20] kvm/vmx: Emulate MSR TEST_CTL

From: Xiaoyao Li <xiaoyao.li@linux.intel.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Fenghua Yu <fenghua.yu@intel.com>, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>, H Peter Anvin <hpa@zytor.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Ashok Raj <ashok.raj@intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Kalle Valo <kvalo@codeaurora.org>,
	Michael Chan <michael.chan@broadcom.com>,
	Ravi V Shankar <ravi.v.shankar@intel.com>,
	linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>,
	linux-wireless@vger.kernel.org, netdev@vger.kernel.org,
	kvm@vger.kernel.org
Subject: Re: [PATCH v6 12/20] kvm/vmx: Emulate MSR TEST_CTL
Date: Wed, 10 Apr 2019 13:03:59 +0800	[thread overview]
Message-ID: <9b16b5c5379eb6b5352de26e56b8064a6b6f1aa7.camel@linux.intel.com> (raw)
In-Reply-To: <20190408174805.GJ25880@linux.intel.com>

On Mon, 2019-04-08 at 10:48 -0700, Sean Christopherson wrote:
> On Mon, Apr 08, 2019 at 05:54:25PM +0800, Xiaoyao Li wrote:
> > On Fri, 2019-04-05 at 14:30 +0200, Thomas Gleixner wrote:
> > > On Wed, 3 Apr 2019, Fenghua Yu wrote:
> > > > @@ -1663,6 +1663,12 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu,
> > > > struct
> > > > msr_data *msr_info)
> > > >  	u32 index;
> > > >  
> > > >  	switch (msr_info->index) {
> > > > +	case MSR_TEST_CTL:
> > > > +		if (!msr_info->host_initiated &&
> > > > +		    !(vcpu->arch.core_capability &
> > > > CORE_CAP_SPLIT_LOCK_DETECT))
> > > 
> > > Errm? If the MSR_TEST_CTL is exposed to the guest via CPUID then the
> > > rdmsr() in the guest is not supposed to fail just because
> > > CORE_CAP_SPLIT_LOCK_DETECT is not set.
> > 
> > you're right.
> > 
> > > vmx->msr_test_ctl holds the proper value, which is either 0 or
> > > CORE_CAP_SPLIT_LOCK_DETECT until more bits are supported.
> > > 
> > > So the chek needs to be guest_cpuid_has(X86_FEATURE_CORE_CAPABILITY).
> > 
> > I don't think so. There is no dependecy between
> > guest_cpuid_has(X86_FEATURE_CORE_CAPABILITY) and MSR_TEST_CTL.
> > guest_cpuid_has(X86_FEATURE_CORE_CAPABILITY) only indicates that guest has
> > MSR
> > CORE_CAPABILITY.
> > 
> > Due to the fact that MSR_TEST_CTL is emulated with vmx->msr_test_ctl. I
> > think it
> > 's ok to always let userspace or guest to read MSR_TEST_CTL, it just returns
> > the
> > emulated value. Like you said, " vmx->msr_test_ctl holds the proper value,
> > which
> > is either 0 or CORE_CAP_SPLIT_LOCK_DETECT until more bits are supported."
> 
> Assuming the next version implements "vmx->msr_test_ctl_mask", KVM should
> inject #GP if the guest attempts RDMSR(MSR_TEST_CTL) and the mask is zero.
> It stands to reason that a kernel can only reasonably assume the MSR exists
> if the (virtual) CPU supports at least one feature enabled via MSR_TEST_CTL.

It makes sense to me. Thanks for your reminder.
Will apply it in next version.