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

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Date:   Thu, 16 Dec 2021 10:21:48 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.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()

> 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. 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?

Thanks
Kevin