linux-kernel - Re: x86 memcpy performance

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Date:	Mon, 15 Aug 2011 10:59:21 -0400
From:	Andy Lutomirski <luto@....EDU>
To:	Borislav Petkov <bp@...en8.de>
CC:	melwyn lobo <linux.melwyn@...il.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	borislav.petkov@....com
Subject: Re: x86 memcpy performance

On 08/15/2011 10:55 AM, Borislav Petkov wrote:
> On Mon, 15 August, 2011 3:27 pm, melwyn lobo wrote:
>> Hi,
>> Was on a vacation for last two days. Thanks for the good insights into
>> the issue.
>> Ingo, unfortunately the data we have is on a soon to be released
>> platform and strictly confidential at this stage.
>>
>> Boris, thanks for the patch. On seeing your patch:
>> +void *__sse_memcpy(void *to, const void *from, size_t len)
>> +{
>> +       unsigned long src = (unsigned long)from;
>> +       unsigned long dst = (unsigned long)to;
>> +       void *p = to;
>> +       int i;
>> +
>> +       if (in_interrupt())
>> +               return __memcpy(to, from, len)
>> So what is the reason we cannot use sse_memcpy in interrupt context.
>> (fpu registers not saved ? )
>
> Because, AFAICT, when we handle an #NM exception while running
> sse_memcpy in an IRQ handler, we might need to allocate FPU save state
> area, which in turn, can sleep. Then, we might get another IRQ while
> sleeping and we should be deadlocked.
>
> But let me stress on the "AFAICT" above, someone who actually knows the
> FPU code should correct me if I'm missing something.

I don't think you ever get #NM as a result of kernel_fpu_begin, but you 
can certainly have problems when kernel_fpu_begin nests by accident. 
There's irq_fpu_usable() for this.

(irq_fpu_usable() reads cr0 sometimes and I suspect it can be slow.)

--Andy
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