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Date: Fri, 27 Mar 2015 13:16:45 +0100 From: Ingo Molnar <mingo@...nel.org> To: Denys Vlasenko <dvlasenk@...hat.com> Cc: Brian Gerst <brgerst@...il.com>, 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>, Linus Torvalds <torvalds@...ux-foundation.org> Subject: Re: [PATCH] x86/asm/entry/64: better check for canonical address * Denys Vlasenko <dvlasenk@...hat.com> wrote: > > 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! :( Ugh, that's really expensive! Why is that so? Same-ring irqs are supposedly a lot simpler. Thanks, Ingo -- 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/
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