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

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Date:   Thu, 16 Dec 2021 11:24:23 +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 11:21, Tian, Kevin wrote:
>> From: Paolo Bonzini
>> Sent: Wednesday, December 15, 2021 6:41 PM
>>
>> There's also another important thing that hasn't been mentioned so far:
>> KVM_GET_SUPPORTED_CPUID should _not_ include the dynamic bits in
>> CPUID[0xD] if they have not been requested with prctl.  It's okay to
>> return the AMX bit, but not the bit in CPUID[0xD].
> 
> There is no vcpu in this ioctl, thus we cannot check vcpu->arch.guest_fpu.perm.
> 
> This then requires exposing xstate_get_guest_group_perm() to KVM.

Right, this is a generic /dev/kvm ioctl therefore it has to check the 
process state.

> Thomas, are you OK with this change given Paolo's ask? v1 included
> this change but it was not necessary at the moment:
> 
> 	https://lore.kernel.org/lkml/87lf0ot50q.ffs@tglx/
> 
> and Paolo, do we want to document that prctl() must be done before
> calling KVM_GET_SUPPORTED_CPUID? If yes, where is the proper location?

You can document it under the KVM_GET_SUPPORTED_CPUID ioctl.

Paolo