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From: Denys Vlasenko <dvlasenk@redhat.com>
To: David Laight <David.Laight@ACULAB.COM>,
	"hpa@zytor.com" <hpa@zytor.com>,
	"torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
	"luto@kernel.org" <luto@kernel.org>,
	"mingo@kernel.org" <mingo@kernel.org>,
	"bp@alien8.de" <bp@alien8.de>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux@dominikbrodowski.net" <linux@dominikbrodowski.net>,
	"brgerst@gmail.com" <brgerst@gmail.com>,
	"peterz@infradead.org" <peterz@infradead.org>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"jpoimboe@redhat.com" <jpoimboe@redhat.com>,
	"linux-tip-commits@vger.kernel.org"
	<linux-tip-commits@vger.kernel.org>
Subject: Re: [tip:x86/pti] x86/entry/64: Introduce the PUSH_AND_CLEAN_REGS macro
Date: Mon, 12 Feb 2018 14:43:50 +0100	[thread overview]
Message-ID: <6217451a-21ea-5fc3-54f7-1e333452dcda@redhat.com> (raw)
In-Reply-To: <1b5552f1231b4c9b867a17d0c5c594bb@AcuMS.aculab.com>

On 02/12/2018 02:36 PM, David Laight wrote:
> From: Denys Vlasenko
>> Sent: 12 February 2018 13:29
> ...
>>>
>>> x86/entry/64: Introduce the PUSH_AND_CLEAN_REGS macro
>>>
>>> Those instances where ALLOC_PT_GPREGS_ON_STACK is called just before
>>> SAVE_AND_CLEAR_REGS can trivially be replaced by PUSH_AND_CLEAN_REGS.
>>> This macro uses PUSH instead of MOV and should therefore be faster, at
>>> least on newer CPUs.
> ...
>>> Link: http://lkml.kernel.org/r/20180211104949.12992-5-linux@dominikbrodowski.net
>>> Signed-off-by: Ingo Molnar <mingo@kernel.org>
>>> ---
>>>    arch/x86/entry/calling.h  | 36 ++++++++++++++++++++++++++++++++++++
>>>    arch/x86/entry/entry_64.S |  6 ++----
>>>    2 files changed, 38 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
>>> index a05cbb8..57b1b87 100644
>>> --- a/arch/x86/entry/calling.h
>>> +++ b/arch/x86/entry/calling.h
>>> @@ -137,6 +137,42 @@ For 32-bit we have the following conventions - kernel is built with
>>>    	UNWIND_HINT_REGS offset=\offset
>>>    	.endm
>>>
>>> +	.macro PUSH_AND_CLEAR_REGS
>>> +	/*
>>> +	 * Push registers and sanitize registers of values that a
>>> +	 * speculation attack might otherwise want to exploit. The
>>> +	 * lower registers are likely clobbered well before they
>>> +	 * could be put to use in a speculative execution gadget.
>>> +	 * Interleave XOR with PUSH for better uop scheduling:
>>> +	 */
>>> +	pushq   %rdi		/* pt_regs->di */
>>> +	pushq   %rsi		/* pt_regs->si */
>>> +	pushq   %rdx		/* pt_regs->dx */
>>> +	pushq   %rcx		/* pt_regs->cx */
>>> +	pushq   %rax		/* pt_regs->ax */
>>> +	pushq   %r8		/* pt_regs->r8 */
>>> +	xorq    %r8, %r8	/* nospec   r8 */
>>
>> xorq's are slower than xorl's on Silvermont/Knights Landing.
>> I propose using xorl instead.
> 
> Does using movq to copy the first zero to the other registers make
> the code any faster?
> 
> ISTR mov reg-reg is often implemented as a register rename rather than an
> alu operation.

xorl is implemented in register rename as well. Just, for some reason,
xorq did not get the same treatment on those CPUs.

  reply	other threads:[~2018-02-12 13:43 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-02-11 10:49 [PATCH v3 0/7] x86/entry: simplify and unify SAVE/POP_REGS Dominik Brodowski
2018-02-11 10:49 ` [PATCH v3 1/7] x86/entry: merge SAVE_C_REGS and SAVE_EXTRA_REGS, remove unused extensions Dominik Brodowski
2018-02-12 10:15   ` [tip:x86/pti] x86/entry/64: Merge " tip-bot for Dominik Brodowski
2018-02-13  9:00   ` tip-bot for Dominik Brodowski
2018-02-11 10:49 ` [PATCH v3 2/7] x86/entry: merge POP_C_REGS and POP_EXTRA_REGS Dominik Brodowski
2018-02-12 10:16   ` [tip:x86/pti] x86/entry/64: Merge the POP_C_REGS and POP_EXTRA_REGS macros into a single POP_REGS macro tip-bot for Dominik Brodowski
2018-02-13  9:01   ` tip-bot for Dominik Brodowski
2018-02-11 10:49 ` [PATCH v3 3/7] x86/entry: interleave XOR register clearing with PUSH instructions Dominik Brodowski
2018-02-12 10:16   ` [tip:x86/pti] x86/entry/64: Interleave " tip-bot for Dominik Brodowski
2018-02-13  9:01   ` tip-bot for Dominik Brodowski
2018-02-11 10:49 ` [PATCH v3 4/7] x86/entry: introduce PUSH_AND_CLEAN_REGS Dominik Brodowski
2018-02-12 10:17   ` [tip:x86/pti] x86/entry/64: Introduce the PUSH_AND_CLEAN_REGS macro tip-bot for Dominik Brodowski
2018-02-12 13:29     ` Denys Vlasenko
2018-02-12 13:36       ` David Laight
2018-02-12 13:43         ` Denys Vlasenko [this message]
2018-02-12 16:51       ` Linus Torvalds
2018-02-13  9:01   ` tip-bot for Dominik Brodowski
2018-02-11 10:49 ` [PATCH v3 5/7] x86/entry: use PUSH_AND_CLEAN_REGS in more cases Dominik Brodowski
2018-02-12 10:17   ` [tip:x86/pti] x86/entry/64: Use " tip-bot for Dominik Brodowski
2018-02-13  9:02   ` tip-bot for Dominik Brodowski
2018-02-11 10:49 ` [PATCH v3 6/7] x86/entry: get rid of ALLOC_PT_GPREGS_ON_STACK and SAVE_AND_CLEAR_REGS Dominik Brodowski
2018-02-12 10:18   ` [tip:x86/pti] x86/entry/64: Get rid of the ALLOC_PT_GPREGS_ON_STACK and SAVE_AND_CLEAR_REGS macros tip-bot for Dominik Brodowski
2018-02-13  9:02   ` tip-bot for Dominik Brodowski
2018-02-11 10:49 ` [PATCH v3 7/7] x86/entry: indent PUSH_AND_CLEAR_REGS and POP_REGS properly Dominik Brodowski
2018-02-12 10:18   ` [tip:x86/pti] x86/entry/64: Indent " tip-bot for Dominik Brodowski
2018-02-12 20:13     ` [PATCH] x86/entry/64: Remove unused icebp macro Borislav Petkov
2018-02-13  7:36       ` Ingo Molnar
2018-02-13  9:04       ` [tip:x86/pti] x86/entry/64: Remove the unused 'icebp' macro tip-bot for Borislav Petkov
2018-02-13  9:03   ` [tip:x86/pti] x86/entry/64: Indent PUSH_AND_CLEAR_REGS and POP_REGS properly tip-bot for Dominik Brodowski
2018-02-11 10:49 ` [PATCH v3 8/7] TESTING_ONLY x86/entry: reduce static footprint of idtentry Dominik Brodowski
2018-02-12  9:37   ` Ingo Molnar
2018-02-12 19:17     ` Linus Torvalds
2018-02-13 12:41       ` Ingo Molnar

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