From: "H. Peter Anvin" <hpa@zytor.com>
To: Borislav Petkov <bp@alien8.de>
Cc: X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 3/3] x86, FPU: Do not use static_cpu_has before alternatives
Date: Mon, 29 Apr 2013 11:58:49 -0700 [thread overview]
Message-ID: <517EC2E9.4030303@zytor.com> (raw)
In-Reply-To: <20130429185123.GB7049@pd.tnic>
On 04/29/2013 11:51 AM, Borislav Petkov wrote:
> On Mon, Apr 29, 2013 at 08:42:30AM -0700, H. Peter Anvin wrote:
>> I *was* considering adding static_cpu_has_safe() at some point which
>> would have a three-state jump, with the default (pre-alternatives)
>> jump pointing to dynamic detection code.
>
> Actually, if we teach __static_cpu_has to do something like
> ALTERNATIVE_JUMP arch/x86/lib/copy_user_64.S but make the second
> alternative insn alt2 be none, i.e. no replacement, we can have:
>
> * pre-alternatives: JMP dynamic_detection
> * post-alternatives:
> - feature present: delete JMP
> - feature absent: s/dynamic_detection/t_no/, i.e., patch only the label.
That is what I was referring to, yes.
> And even though asm goto supports multiple labels, we need to be able
> to either patch the label only or patch out the whole instruction -
> otherwise we'll be adding additional NOP bytes.
This doesn't seem to matter.
> I wonder if it would make sense to teach the alternatives to skip the
> opcode when patching so that we can say: "we only want to patch the
> label so we're patching in the offset now but leaving the single JMP
> opcode in there."
>
> But for that we either need flags in struct alt_instr or do something
> ad-hoc apply_alternatives already does for relative jumps (0xe8).
>
>> This might be useful here, on the other hand, perhaps it is acceptable
>> for use_eager_fpu() to be initially false?
>
> Hmm, I don't know, FPU code is crazy. OTOH, does CR0.TS even matter on
> non-lazy FPU restore machines?
Yes, CR0.TS should be zero, or we'll get #NM traps when user space tries
to access the FPU. For lazy restore CR0.TS should be set so we get the
#NM trap and can restore the FPU.
-hpa
next prev parent reply other threads:[~2013-04-29 18:59 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-04-29 14:04 [PATCH 0/3] FPU detection in C, second try Borislav Petkov
2013-04-29 14:04 ` [PATCH 1/3] x86: Get rid of ->hard_math and all the FPU asm fu Borislav Petkov
2013-04-29 14:04 ` [PATCH 2/3] x86: Sanity-check static_cpu_has usage Borislav Petkov
2013-04-29 15:38 ` H. Peter Anvin
2013-04-29 14:04 ` [PATCH 3/3] x86, FPU: Do not use static_cpu_has before alternatives Borislav Petkov
2013-04-29 15:42 ` H. Peter Anvin
2013-04-29 18:51 ` Borislav Petkov
2013-04-29 18:58 ` H. Peter Anvin [this message]
-- strict thread matches above, loose matches on Subject: below --
2013-04-08 15:57 [PATCH 0/4] x86: FPU detection in C Borislav Petkov
2013-04-08 15:57 ` [PATCH 1/4] x86: Get rid of ->hard_math and all the FPU asm fu Borislav Petkov
2013-04-08 16:26 ` [tip:x86/cpu] " tip-bot for H. Peter Anvin
2013-06-12 20:48 ` [tip:x86/fpu] " tip-bot for H. Peter Anvin
2013-04-08 15:57 ` [PATCH 2/4] x86: Fold-in trivial check_config function Borislav Petkov
2013-04-08 16:27 ` [tip:x86/cpu] " tip-bot for Borislav Petkov
2013-04-16 13:51 ` tip-bot for Borislav Petkov
2013-04-08 15:57 ` [PATCH 3/4] x86, AMD: Correct {rd,wr}msr_amd_safe warnings Borislav Petkov
2013-04-08 16:28 ` [tip:x86/cpu] " tip-bot for Borislav Petkov
2013-04-16 13:53 ` tip-bot for Borislav Petkov
2013-04-08 15:57 ` [PATCH 4/4] x86, CPU, AMD: Drop useless label Borislav Petkov
2013-04-08 16:29 ` [tip:x86/cpu] " tip-bot for Borislav Petkov
2013-04-16 13:54 ` tip-bot for Borislav Petkov
2013-04-10 11:08 ` [PATCH 0/4] x86: FPU detection in C Ingo Molnar
2013-04-10 12:24 ` Borislav Petkov
2013-04-10 12:25 ` Ingo Molnar
2013-04-10 13:32 ` Borislav Petkov
2013-04-10 15:35 ` H. Peter Anvin
2013-04-10 16:11 ` Borislav Petkov
2013-04-10 21:29 ` [PATCH] x86, FPU: Fix FPU initialization Borislav Petkov
2013-04-11 12:09 ` Ingo Molnar
2013-04-11 14:23 ` Borislav Petkov
2013-04-11 19:26 ` H. Peter Anvin
2013-04-11 20:23 ` Borislav Petkov
2013-04-12 5:32 ` Ingo Molnar
2013-04-12 5:34 ` H. Peter Anvin
2013-04-12 9:47 ` Borislav Petkov
2013-04-12 11:26 ` Borislav Petkov
2013-04-15 10:08 ` Ingo Molnar
2013-04-15 10:17 ` Borislav Petkov
2013-04-15 10:18 ` Ingo Molnar
2013-04-15 15:54 ` Borislav Petkov
2013-04-15 22:04 ` Borislav Petkov
2013-04-16 9:25 ` Ingo Molnar
2013-04-16 9:48 ` Borislav Petkov
2013-04-16 10:08 ` [GIT PULL] Rebase tip:x86/cpu Borislav Petkov
2013-04-16 11:35 ` Ingo Molnar
2013-04-16 11:33 ` [PATCH] x86, FPU: Fix FPU initialization Ingo Molnar
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