linux-kernel - RE: [PATCH 10/19] kvm: x86: Emulate WRMSR of guest IA32

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Message-ID: <MWHPR11MB1245F7730D9BF0DA251D302DA9749@MWHPR11MB1245.namprd11.prod.outlook.com>
Date:   Mon, 13 Dec 2021 07:51:27 +0000
From:   "Liu, Jing2" <jing2.liu@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        "Zhong, Yang" <yang.zhong@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
CC:     "Christopherson,, Sean" <seanjc@...gle.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "jing2.liu@...ux.intel.com" <jing2.liu@...ux.intel.com>,
        "Wang, Wei W" <wei.w.wang@...el.com>,
        "Zeng, Guang" <guang.zeng@...el.com>
Subject: RE: [PATCH 10/19] kvm: x86: Emulate WRMSR of guest IA32_XFD

On 12/11/2021 12:02 AM, Paolo Bonzini wrote:
> 
> Also:
> 
> On 12/8/21 01:03, Yang Zhong wrote:
> >
> > +		if (!guest_cpuid_has(vcpu, X86_FEATURE_XFD))
> > +			return 1;
> 
> This should allow msr->host_initiated always (even if XFD is not part of
> CPUID). 
Thanks Paolo.

msr->host_initiated handling would be added in next version.

I'd like to ask why always allow msr->host_initiated even if XFD is not part of
CPUID, although guest doesn't care that MSR?  We found some MSRs
 (e.g. MSR_AMD64_OSVW_STATUS and MSR_AMD64_OSVW_ID_LENGTH ) 
are specially handled so would like to know the consideration of allowing
msr->host_initiated.

if (!msr_info->host_initiated && !guest_cpuid_has(vcpu, X86_FEATURE_XFD))
        return 1;


 However, if XFD is nonzero and kvm_check_guest_realloc_fpstate
> returns true, then it should return 1.
> 
If XFD is nonzero, kvm_check_guest_realloc_fpstate() won't return true. So
may not need this check here?

Thanks,
Jing

> 
> Paolo