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

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Date:   Wed, 29 Nov 2023 22:12:59 +0800
From:   "Yang, Weijiang" <weijiang.yang@...el.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
CC:     "Hansen, Dave" <dave.hansen@...el.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "john.allen@....com" <john.allen@....com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Gao, Chao" <chao.gao@...el.com>,
        "mlevitsk@...hat.com" <mlevitsk@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 05/26] x86/fpu/xstate: Introduce fpu_guest_cfg for
 guest FPU configuration

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() .