linux-kernel - Re: [PATCH 4/4] x86/fpu: don't abuse drop_init_fpu() in flush

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20150313161958.GI31998@pd.tnic>
Date:	Fri, 13 Mar 2015 17:19:58 +0100
From:	Borislav Petkov <bp@...e.de>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Dave Hansen <dave.hansen@...el.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Riikonen <priikone@....fi>,
	Rik van Riel <riel@...hat.com>,
	Suresh Siddha <sbsiddha@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Yu, Fenghua" <fenghua.yu@...el.com>,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>
Subject: Re: [PATCH 4/4] x86/fpu: don't abuse drop_init_fpu() in
 flush_thread()

On Fri, Mar 13, 2015 at 03:55:42PM +0100, Oleg Nesterov wrote:
> But please look at drop_init_fpu(). If eagerfpu == F it calls drop_fpu() and
> this is what we need. flush_thread() already has the "if (!use_eager_fpu())",
> we can shift drop_fpu() there.
> 
> Otherwise, if eagerfpu == T, drop_init_fpu() does restore_init_xstate() and
> this just burns CPU. Until flush_thread user_has_fpu/used_math this state
> restore_init_xstate() is pointless, this state will be lost after preemption.

Yeah, I was wondering why that's there.

One example where drop_init_fpu() seems to make sense is
__kernel_fpu_end(): kernel is done with FPU and current was using the
FPU prior so let's restore it for the eagerfpu case.

Thanks.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
--
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/