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From: "Radim Krčmář" <rkrcmar@redhat.com>
To: Xiaoyao Li <xiaoyao.li@linux.intel.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 17:50:38 +0200
Message-ID: <20190617155038.GA13955@flask> (raw)
In-Reply-To: <ea1fc40b-8f80-d5f0-6c97-adb245599e07@linux.intel.com>

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.)

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 [this message]
2019-06-17 15:54         ` Xiaoyao Li
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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