Re: [PATCH v5 3/7] KVM: X86: Expose IA32_PKRS MSR

From: Chenyi Qiang <chenyi.qiang@intel.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	Xiaoyao Li <xiaoyao.li@intel.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 3/7] KVM: X86: Expose IA32_PKRS MSR
Date: Wed, 10 Nov 2021 08:56:41 +0800	[thread overview]
Message-ID: <0b1ac54e-5706-4864-a4a9-1d1a2cff354a@intel.com> (raw)
In-Reply-To: <YYqT/cOm3Psf1gj1@google.com>

On 11/9/2021 11:30 PM, Sean Christopherson wrote:
> On Tue, Nov 09, 2021, Chenyi Qiang wrote:
>>
>> On 11/9/2021 1:44 AM, Sean Christopherson wrote:
>>> Hrm.  Ideally this would be open coded in vmx_set_msr().  Long term, the RESET/INIT
>>> paths should really treat MSR updates as "normal" host_initiated writes instead of
>>> having to manually handle every MSR.
>>>
>>> That would be a bit gross to handle in vmx_vcpu_reset() since it would have to
>>> create a struct msr_data (because __kvm_set_msr() isn't exposed to vendor code),
>>> but since vcpu->arch.pkrs is relevant to the MMU I think it makes sense to
>>> initiate the write from common x86.
>>>
>>> E.g. this way there's not out-of-band special code, vmx_vcpu_reset() is kept clean,
>>> and if/when SVM gains support for PKRS this particular path Just Works.  And it would
>>> be an easy conversion for my pipe dream plan of handling MSRs at RESET/INIT via a
>>> list of MSRs+values.
>>>
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index ac83d873d65b..55881d13620f 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -11147,6 +11147,9 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>>>           kvm_set_rflags(vcpu, X86_EFLAGS_FIXED);
>>>           kvm_rip_write(vcpu, 0xfff0);
>>>
>>> +       if (kvm_cpu_cap_has(X86_FEATURE_PKS))
>>> +               __kvm_set_msr(vcpu, MSR_IA32_PKRS, 0, true);
>>> +
>>
>> Got it. In addition, is it necessary to add on-INIT check? like:
>>
>> if (kvm_cpu_cap_has(X86_FEATURE_PKS) && !init_event)
>> 	__kvm_set_msr(vcpu, MSR_IA32_PKRS, 0, true);
>>
>> PKRS should be preserved on INIT, not cleared. The SDM doesn't make this
>> clear either.
> 
> Hmm, but your cover letter says:
> 
>    To help patches review, one missing info in SDM is that PKSR will be
>    cleared on Powerup/INIT/RESET, which should be listed in Table 9.1
>    "IA-32 and Intel 64 Processor States Following Power-up, Reset, or INIT"
> 
> Which honestly makes me a little happy because I thought I was making stuff up
> for a minute :-)
> 
> So which is it?

Sorry about the confusion. PKRS is preserved on INIT. I tried to correct 
my statement in previous ping mail but seems not so obvious.

> 