linux-kernel - Re: [PATCH RESEND v3 2/3] KVM: vmx: Emulate MSR IA32_UMWAIT

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Message-ID: <20190617155038.GA13955@flask>
Date:   Mon, 17 Jun 2019 17:50:38 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Xiaoyao Li <xiaoyao.li@...ux.intel.com>
Cc:     Tao Xu <tao3.xu@...el.com>, pbonzini@...hat.com, corbet@....net,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        sean.j.christopherson@...el.com, fenghua.yu@...el.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        jingqi.liu@...el.com
Subject: Re: [PATCH RESEND v3 2/3] KVM: vmx: Emulate MSR IA32_UMWAIT_CONTROL

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.