linux-kernel - Re: [PATCH v3 1/4] x86/clear_page: extend clear

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <878qo234rj.fsf@oracle.com>
Date: Mon, 14 Apr 2025 12:46:40 -0700
From: Ankur Arora <ankur.a.arora@...cle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...nel.org>, Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        torvalds@...ux-foundation.org, akpm@...ux-foundation.org, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, mingo@...hat.com,
        luto@...nel.org, paulmck@...nel.org, rostedt@...dmis.org,
        tglx@...utronix.de, willy@...radead.org, jon.grimm@....com,
        bharata@....com, raghavendra.kt@....com, boris.ostrovsky@...cle.com,
        konrad.wilk@...cle.com
Subject: Re: [PATCH v3 1/4] x86/clear_page: extend clear_page*() for
 multi-page clearing


Peter Zijlstra <peterz@...radead.org> writes:

> On Mon, Apr 14, 2025 at 08:32:29AM +0200, Ingo Molnar wrote:

[ ... ]

>> > -SYM_TYPED_FUNC_START(clear_page_orig)
>> > +/*
>> > + * Original page zeroing loop.
>> > + * Input:
>> > + * %rdi	- destination
>> > + * %esi	- length
>> > + *
>> > + * Clobbers: %rax, %rcx, %rflags
>> > + */
>> > +SYM_TYPED_FUNC_START(clear_pages_orig)
>> > +	movl   %esi, %ecx
>> >  	xorl   %eax,%eax
>> > -	movl   $4096/64,%ecx
>> > +	shrl   $6,%ecx
>>
>> So if the natural input parameter is RCX, why is this function using
>> RSI as the input 'length' parameter? Causes unnecessary register
>> shuffling.
>
> This symbol is written as a C function with C calling convention, even
> though it is only meant to be called from that clear_page() alternative.
>
> If we want to go change all this, then we should go do the same we do
> for __clear_user() and write it thusly:
>
> 	asm volatile(ALTERNATIVE("rep stosb",
> 				 "call rep_stos_alternative", ALT_NOT(X86_FEATURE_FSRS)
> 				 : "+c" (size), "+D" (addr), ASM_CALL_CONSTRAINT
> 				 : "a" (0))
>
> And forget about all those clear_page_*() thingies.

Yeah, this makes sense. We don't call any of the clear_pages_*() variants
from anywhere else.

clear_pages_rep() and clear_pages_erms() are trivial enough to be
inlined in the ALTERNATIVE as well.

Thanks!

--
ankur