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From: Xiaoyao Li <xiaoyao.li@linux.intel.com>
To: "Radim Krčmář" <rkrcmar@redhat.com>
Cc: Tao Xu <tao3.xu@intel.com>,
	pbonzini@redhat.com, corbet@lwn.net, tglx@linutronix.de,
	mingo@redhat.com, bp@alien8.de, hpa@zytor.com,
	sean.j.christopherson@intel.com, fenghua.yu@intel.com,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	jingqi.liu@intel.com
Subject: Re: [PATCH RESEND v3 2/3] KVM: vmx: Emulate MSR IA32_UMWAIT_CONTROL
Date: Mon, 17 Jun 2019 23:54:42 +0800
Message-ID: <5f34bd4b-b3d1-1950-e4d5-8e65c3809ab1@linux.intel.com> (raw)
In-Reply-To: <20190617155038.GA13955@flask>



On 6/17/2019 11:50 PM, Radim Krčmář wrote:
> 2019-06-17 14:31+0800, Xiaoyao Li:
>> On 6/17/2019 11:32 AM, Xiaoyao Li wrote:
>>> On 6/16/2019 5:55 PM, Tao Xu wrote:
>>>> +    if (vmx->msr_ia32_umwait_control != host_umwait_control)
>>>> +        add_atomic_switch_msr(vmx, MSR_IA32_UMWAIT_CONTROL,
>>>> +                      vmx->msr_ia32_umwait_control,
>>>> +                      host_umwait_control, false);
>>>
>>> The bit 1 is reserved, at least, we need to do below to ensure not
>>> modifying the reserved bit:
>>>
>>>       guest_val = (vmx->msr_ia32_umwait_control & ~BIT_ULL(1)) |
>>>               (host_val & BIT_ULL(1))
>>>
>>
>> I find a better solution to ensure reserved bit 1 not being modified in
>> vmx_set_msr() as below:
>>
>> 	if((data ^ umwait_control_cached) & BIT_ULL(1))
>> 		return 1;
> 
> We could just be checking
> 
> 	if (data & BIT_ULL(1))
> 
> because the guest cannot change its visible reserved value and KVM
> currently initializes the value to 0.
> 
> The arch/x86/kernel/cpu/umwait.c series assumes that the reserved bit
> is 0 (hopefully deliberately) and I would do the same in KVM as it
> simplifies the logic.  (We don't have to even think about migrations
> between machines with a different reserved value and making it play
> nicely with possible future implementations of that bit.)
> 

Got it, thanks.

> Thanks.
> 

  reply index

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-06-16  9:55 [PATCH RESEND v3 0/3] KVM: x86: Enable user wait instructions Tao Xu
2019-06-16  9:55 ` [PATCH RESEND v3 1/3] KVM: x86: add support for " Tao Xu
2019-06-16  9:55 ` [PATCH RESEND v3 2/3] KVM: vmx: Emulate MSR IA32_UMWAIT_CONTROL Tao Xu
2019-06-17  3:32   ` Xiaoyao Li
2019-06-17  6:31     ` Xiaoyao Li
2019-06-17 15:50       ` Radim Krčmář
2019-06-17 15:54         ` Xiaoyao Li [this message]
2019-06-18  2:40         ` Tao Xu
2019-06-18  3:04     ` Tao Xu
2019-06-16  9:55 ` [PATCH RESEND v3 3/3] KVM: vmx: handle vm-exit for UMWAIT and TPAUSE Tao Xu

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