From:   Thomas Gleixner <tglx@linutronix.de>
To:     "Tian, Kevin" <kevin.tian@intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        "Wang, Wei W" <wei.w.wang@intel.com>,
        "quintela@redhat.com" <quintela@redhat.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
        Jing Liu <jing2.liu@linux.intel.com>,
        "Zhong, Yang" <yang.zhong@intel.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "Christopherson,, Sean" <seanjc@google.com>,
        "Nakajima, Jun" <jun.nakajima@intel.com>,
        "Zeng, Guang" <guang.zeng@intel.com>
Subject: RE: [patch 5/6] x86/fpu: Provide fpu_update_guest_xcr0/xfd()
In-Reply-To: <BN9PR11MB52761B401F752514B22A23768C779@BN9PR11MB5276.namprd11.prod.outlook.com>
References: <20211214022825.563892248@linutronix.de>
 <20211214024948.048572883@linutronix.de>
 <854480525e7f4f3baeba09ec6a864b80@intel.com> <87zgp3ry8i.ffs@tglx>
 <b3ac7ba45c984cf39783e33e0c25274d@intel.com> <87r1afrrjx.ffs@tglx>
 <87k0g7qa3t.fsf@secure.mitica> <87k0g7rkwj.ffs@tglx>
 <878rwm7tu8.fsf@secure.mitica>
 <afeba57f71f742b88aac3f01800086f9@intel.com> <878rwmrxgb.ffs@tglx>
 <a4fbf9f8-8876-f58c-d2b6-15add35bedd0@redhat.com>
 <BN9PR11MB5276E2165EB86520520D54FD8C779@BN9PR11MB5276.namprd11.prod.outlook.com>
 <87fsqslwph.ffs@tglx>
 <BN9PR11MB52761B401F752514B22A23768C779@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Thu, 16 Dec 2021 15:12:53 +0100
Message-ID: <8735msljtm.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
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On Thu, Dec 16 2021 at 09:59, Kevin Tian wrote:
>> From: Thomas Gleixner <tglx@linutronix.de>
>> This can be done simply with the MSR entry/exit controls. No trap
>> required neither for #NM for for XFD_ERR.
>> 
>> VMENTER loads guest state. VMEXIT saves guest state and loads host state
>> (0)
>
> This implies three MSR operations for every vm-exit.
>
> With trap we only need one RDMSR in host #NM handler, one 
> RDMSR/one WRMSR exit in guest #NM handler, which are both rare.
> plus one RDMSR/one WRMSR per vm-exit only if saved xfd_err is 
> non-zero which is again rare.

Fair enough.

>> XFD:     Always guest state
>> 
>> So VMENTER does nothing and VMEXIT either saves guest state and the sync
>> function uses the automatically saved value or you keep the sync
>> function which does the rdmsrl() as is.
>> 
>
> Yes, this is the 3rd open that I asked in another reply. The only restriction
> with this approach is that the sync cost is added also for legacy OS which
> doesn't touch xfd at all. 

You still can make that conditional on the guest XCR0. If guest never
enables the extended bit then neither the #NM trap nor the XFD sync
are required.

But yes, there are too many moving parts here :)

Thanks,

        tglx