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

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Message-ID: <90cb8be18da40c62f6acbf2bee19ec046e122e49.camel@redhat.com>
Date:   Thu, 30 Nov 2023 19:29:13 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Yang, Weijiang" <weijiang.yang@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>
Cc:     "john.allen@....com" <john.allen@....com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Gao, Chao" <chao.gao@...el.com>
Subject: Re: [PATCH v7 05/26] x86/fpu/xstate: Introduce fpu_guest_cfg for
 guest FPU configuration

On Tue, 2023-11-28 at 14:58 +0000, 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?

It is used because permission API is used for guest fpu state as well (for user features),
and it affects two things:

1. If permission is not asked, then KVM will fail to resize the FPU state to match guest CPUID.
2. It will affect output size of the KVM_GET_XSAVE2 ioctl, which outputs buffer similar to
other FPU state buffers exposed to userspace (like one saved on signal stack, or one obtained via ptrace).


Best regards,
	Maxim Levitsky