linux-kernel - Re: [PATCH v7 05/26] x86/fpu/xstate: Introduce fpu_guest

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Message-ID: <a8ee5217a98b261a2eace9a0b543e43796607156.camel@intel.com>
Date:   Wed, 29 Nov 2023 17:08:19 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "Yang, Weijiang" <weijiang.yang@...el.com>
CC:     "Gao, Chao" <chao.gao@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "john.allen@....com" <john.allen@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mlevitsk@...hat.com" <mlevitsk@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>
Subject: Re: [PATCH v7 05/26] x86/fpu/xstate: Introduce fpu_guest_cfg for
 guest FPU configuration

On Wed, 2023-11-29 at 22:12 +0800, Yang, Weijiang wrote:
> On 11/28/2023 10:58 PM, Edgecombe, Rick P wrote:
> > On Fri, 2023-11-24 at 00:53 -0500, Yang Weijiang wrote:
> > > +       /*
> > > +        * Set guest's __user_state_size to
> > > fpu_user_cfg.default_size
> > > so that
> > > +        * existing uAPIs can still work.
> > > +        */
> > > +       fpu->guest_perm.__user_state_size =
> > > fpu_user_cfg.default_size;
> > It seems like an appropriate value, but where does this come into
> > play
> > exactly for guest FPUs?
> 
> I don't see there's special usage of this field for vCPU in VMM
> userspace(QEMU).
> Maybe it's mainly for AMX resulted usespace fault handling? For vCPU
> thread,
> it's only  referenced when AMX is enabled via __xfd_enable_feature()
> .
> 

In that case the "so that existing uAPIs can still work" comment seems
misleading. Maybe "this doesn't come into play for guest FPUs, but set
it to a reasonable value"?