linux-kernel - Re: [PATCH] x86/fpu: Remove dynamic features from xcomp_bv for init

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Date:   Thu, 13 Oct 2022 10:21:28 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Chang S. Bae" <chang.seok.bae@...el.com>,
        "Yao, Yuan" <yuan.yao@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     "x86@...nel.org" <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86/fpu: Remove dynamic features from xcomp_bv for
 init_fpstate

On 10/13/22 09:23, Chang S. Bae wrote:
> 
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -1127,8 +1127,12 @@ void __copy_xstate_to_uabi_buf(struct membuf to,
> struct fpstate *fpstate,
>          * non-compacted format disabled features still occupy state space,
>          * but there is no state to copy from in the compacted
>          * init_fpstate. The gap tracking will zero these states.
> +        *
> +        * In the case of guest fpstate, this user_xfeatures does not
> +        * dynamically reflect the capacity of the XSAVE buffer but
> +        * xfeatures does. So AND them together.
>          */
> -       mask = fpstate->user_xfeatures;
> +       mask = fpstate->user_xfeatures & fpstate->xfeatures;

I'm not sure this is quite right either.

Doesn't kvm expect that all of the ->user_xfeatures will end up being
copied out?  We surely can't copy them from 'fpstate' if the feature
isn't there, but we can't skip them entirely, can we?