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

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Date:   Thu, 16 Dec 2021 22:07:35 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "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 12/16/21 06:36, Tian, Kevin wrote:
> 2) Do expansion at vCPU creation or KVM_ SET_CPUID2?
> 
> If the reallocation concept is still kept, then we feel doing expansion in
> KVM_SET_CPUID2 makes slightly more sense. There is no functional
> difference between two options since the guest is not running at this
> point. And in general Qemu should set prctl according to the cpuid bits.
> But since anyway we still need to check guest cpuid against guest perm in
> KVM_SET_CPUID2, it reads clearer to expand the buffer only after this
> check is passed.

Yes, that makes sense to me as well.  In principle userspace could call 
prctl only after KVM_CREATE_VCPU.

> 
> One option is to always disable WRMSR interception once 
> KVM_SET_CPUID2 succeeds, with the cost of one RDMSR per vm-exit. 
> But doing so affects legacy OS which even has no XFD logic at all.
> 
> The other option is to continue the current policy i.e. disable write 
> emulation only after the 1st interception of setting XFD to a non-zero 
> value. Then the RDMSR cost is added only for guest which supports XFD.

For this I suggest to implement the current policy, but place it at the 
end of the series so it's easy to drop it.

Paolo