Re: [PATCH] x86/clear_user: Make it faster

From: Borislav Petkov <bp@alien8.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mark Hemment <markhemm@googlemail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	patrice.chotard@foss.st.com,
	Mikulas Patocka <mpatocka@redhat.com>,
	Lukas Czerner <lczerner@redhat.com>,
	Christoph Hellwig <hch@lst.de>,
	"Darrick J. Wong" <djwong@kernel.org>,
	Chuck Lever <chuck.lever@oracle.com>,
	Hugh Dickins <hughd@google.com>,
	patches@lists.linux.dev, Linux-MM <linux-mm@kvack.org>,
	mm-commits@vger.kernel.org, Mel Gorman <mgorman@suse.de>
Subject: Re: [PATCH] x86/clear_user: Make it faster
Date: Tue, 24 May 2022 19:30:43 +0200	[thread overview]
Message-ID: <Yo0WQ0JNElGswnrq@zn.tnic> (raw)
In-Reply-To: <CAHk-=wiJ9xqFrdDJtUD3oc1kCxYcVjLi5BjBY-3NVHgYxYb_HA@mail.gmail.com>

On Tue, May 24, 2022 at 09:51:56AM -0700, Linus Torvalds wrote:
> I can't find anything wrong with this, but who knows what
> patch-blindness I have from looking at a few different versions of it.
> Maybe my eyes just skim over it now.

Same here - I can't look at that code anymore. I'll try to gain some
distance and look at it again later, and do some more extensive testing
too.

> I do note that the clearing of %rax here:
> 
> > +.Lerms_exit:
> > +       xorl %eax,%eax
> > +       RET
> 
> seems to be unnecessary, since %rax is never modified in the path
> leading to this. But maybe just as well just for consistency with the
> cases where it *is* used as a temporary.

Yeah.

> And I still suspect that "copy_to_user()" is *much* more interesting
> than "clear_user()", but I guess we can't inline it anyway due to all
> the other overhead (ie access_ok() and stac/clac).
> 
> And for a plain "call memcpy/memset", we'd need compiler help to do
> this (at a minimum, we'd have to have the compiler use the 'rep
> movs/stos' register logic, and then we could patch things in place
> afterwards, with objtool creating the alternatives section or
> something).

Yeah, I have this on my todo to research them properly. Will report when
I have something.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette