linux-kernel - Re: [PATCH] x86/asm/entry/64: better check for canonical address

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Date:	Fri, 27 Mar 2015 13:14:39 +0100
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>, Brian Gerst <brgerst@...il.com>
CC:	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/asm/entry/64: better check for canonical address

On 03/27/2015 12:34 PM, Ingo Molnar wrote:
> 
> * Brian Gerst <brgerst@...il.com> wrote:
> 
>>> Btw., there's a neat trick we could do: in the HLT, MWAIT and 
>>> ACPI-idle code we could attempt to set up RCX to match RIP, to 
>>> trigger this optimization in the common 'irq interrupted the idle 
>>> task' case?
>>
>> sysret only returns to CPL3.
> 
> Indeed, an IRET ought to be pretty cheap for same-ring interrupt 
> returns in any case.

Unfortunately, it is not. Try attached program.

On this CPU, 1 ns ~= 3 cycles.

$ ./timing_test64 callret
10000 loops in 0.00008s = 7.87 nsec/loop for callret
100000 loops in 0.00076s = 7.56 nsec/loop for callret
1000000 loops in 0.00548s = 5.48 nsec/loop for callret
10000000 loops in 0.02882s = 2.88 nsec/loop for callret
100000000 loops in 0.18334s = 1.83 nsec/loop for callret
200000000 loops in 0.36051s = 1.80 nsec/loop for callret
400000000 loops in 0.71632s = 1.79 nsec/loop for callret

Near call + near ret = 5 cycles

$ ./timing_test64 lret
10000 loops in 0.00034s = 33.95 nsec/loop for lret
100000 loops in 0.00328s = 32.83 nsec/loop for lret
1000000 loops in 0.04541s = 45.41 nsec/loop for lret
10000000 loops in 0.32130s = 32.13 nsec/loop for lret
20000000 loops in 0.64191s = 32.10 nsec/loop for lret

push my_cs + push next_label + far ret = ~90 cycles

$ ./timing_test64 iret
10000 loops in 0.00344s = 343.90 nsec/loop for iret
100000 loops in 0.01890s = 188.97 nsec/loop for iret
1000000 loops in 0.08228s = 82.28 nsec/loop for iret
10000000 loops in 0.77910s = 77.91 nsec/loop for iret

This is the "same-ring interrupt return". ~230 cycles!  :(



View attachment "timing_test.c" of type "text/x-csrc" (6920 bytes)