linux-kernel - RE: [patch 5/6] x86/fpu: Provide fpu_update_guest

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Date:   Thu, 16 Dec 2021 15:12:53 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Wang, Wei W" <wei.w.wang@...el.com>,
        "quintela@...hat.com" <quintela@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Jing Liu <jing2.liu@...ux.intel.com>,
        "Zhong, Yang" <yang.zhong@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "Zeng, Guang" <guang.zeng@...el.com>
Subject: RE: [patch 5/6] x86/fpu: Provide fpu_update_guest_xcr0/xfd()

On Thu, Dec 16 2021 at 09:59, Kevin Tian wrote:
>> From: Thomas Gleixner <tglx@...utronix.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