linux-kernel - Re: [PATCH 24/27] x86/fpu: Add a fastpath to __fpu__restore

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Date:   Fri, 12 Apr 2019 19:27:30 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 24/27] x86/fpu: Add a fastpath to __fpu__restore_sig()

On 2019-04-12 19:17:59 [+0200], Borislav Petkov wrote:
> > @@ -327,7 +327,19 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
> >  		if (ret)
> >  			goto err_out;
> >  		envp = &env;
> > +	} else {
> 
> I've added here:

I would suggest to add it in the last patch once we have the complete
fast path. The other one extended (in case you want to add a comment
there, too).

> +               /*
/**

> +                * Attempt to restore the FPU registers directly from user
> +                * memory. For that to succeed, the user accesses cannot cause

I would say "user access" because it is always a single instruction.

> +                * page faults. If they do, fall back to the slow path below,
> +                * going through the kernel buffer.

"with the enabled page fault handler".

> +                */
> 
> so that it is clear what's happening.
> 
> This function is doing gazillion things again ;-\

It is the old code with disabled page fault handler. But I understand.

Sebastian