From: David Laight <David.Laight@ACULAB.COM>
To: 'Peter Zijlstra' <peterz@infradead.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
Bill Wendling <morbo@google.com>,
Josh Poimboeuf <jpoimboe@redhat.com>,
"x86@kernel.org" <x86@kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"mark.rutland@arm.com" <mark.rutland@arm.com>,
"dvyukov@google.com" <dvyukov@google.com>,
"seanjc@google.com" <seanjc@google.com>,
"pbonzini@redhat.com" <pbonzini@redhat.com>,
"mbenes@suse.cz" <mbenes@suse.cz>,
"llvm@lists.linux.dev" <llvm@lists.linux.dev>,
"linux-toolchains@vger.kernel.org"
<linux-toolchains@vger.kernel.org>
Subject: RE: [PATCH 20/22] x86,word-at-a-time: Remove .fixup usage
Date: Wed, 10 Nov 2021 12:20:47 +0000 [thread overview]
Message-ID: <2734a37ebed2432291345aaa8d9fd47e@AcuMS.aculab.com> (raw)
In-Reply-To: <YYuogZ+2Dnjyj1ge@hirez.programming.kicks-ass.net>
From: Peter Zijlstra
> Sent: 10 November 2021 11:10
>
> On Wed, Nov 10, 2021 at 10:46:42AM +0000, David Laight wrote:
> > From: Peter Zijlstra
> > > Sent: 09 November 2021 21:08
> > ...
> > >
> > > GCC does the same, but I wanted to have the exception stuff be in
> > > .text.cold, but alas it doesn't do that. I left the attribute because of
> > > it's descriptive value.
> > >
> > > > Unless the cold attribute is helping move
> > > > ("shrink-wrap"?) the basic block to a whole other section
> > > > (.text.cold.)?
> > >
> > > I was hoping it would do that, but it doesn't on gcc-11.
> >
> > Wouldn't moving part of a function to .text.cold (or .text.unlikely)
> > generate the same problems with the stack backtrace code as the
> > .text.fixup section you are removing had??
>
> GCC can already split a function into func and func.cold today (or
> worse: func, func.isra.N, func.cold, func.isra.N.cold etc..).
>
> I'm assuming reliable unwind and livepatch know how to deal with this.
They'll have 'proper' function labels at the top - so backtrace
stands a chance.
Indeed you (probably) want it to output "func.irsa.n.cold" rather
than just "func" to help show which copy it is in.
I guess that livepatch will need separate patches for each
version of the function - which might be 'interesting' if
all the copies actually need patching at the same time.
You'd certainly want a warning if there seemed to be multiple
copies of the function.
I'm waiting for the side-channel attack caused by detecting
timing differences caused by TLB misses when speculatively
executing code in the .cold/.unlikely sections.
ISTR recent x86 cpu speculate unknown conditional branches
'randomly' - rather than (say) assuming backwards taken.
So you can't (easily) stop speculative execution into the 'cold'
text.
I don't know if speculative execution will load TLB,
it would speed a lot of code up - with the same downsides
as evicting code from the L1-cache.
A 'half-way house' would be to do the page table walk, but
hold the read value 'pending' the code being needed.
David
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2021-11-10 12:20 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-05 17:10 [PATCH 00/22] x86: Remove anonymous out-of-line fixups Peter Zijlstra
2021-11-05 17:10 ` [PATCH 01/22] bitfield.h: Fix "type of reg too small for mask" test Peter Zijlstra
2021-11-05 17:10 ` [PATCH 02/22] x86,mmx_32: Remove .fixup usage Peter Zijlstra
2021-11-05 17:10 ` [PATCH 03/22] x86,copy_user_64: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 04/22] x86,copy_mc_64: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 05/22] x86,entry_64: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 06/22] x86,entry_32: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 07/22] x86,extable: Extend extable functionality Peter Zijlstra
2021-11-05 17:10 ` [PATCH 08/22] x86,msr: Remove .fixup usage Peter Zijlstra
2021-11-05 17:10 ` [PATCH 09/22] x86,futex: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 10/22] x86,uaccess: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 11/22] x86,xen: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 12/22] x86,fpu: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 13/22] x86,segment: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 14/22] x86,vmx: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 15/22] x86,checksum_32: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 16/22] x86,sgx: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 17/22] x86,kvm: " Peter Zijlstra
2021-11-05 17:10 ` [PATCH 18/22] x86,usercopy_32: Simplify __copy_user_intel_nocache() Peter Zijlstra
2021-11-05 17:10 ` [PATCH 19/22] x86,usercopy: Remove .fixup usage Peter Zijlstra
2021-11-05 17:10 ` [PATCH 20/22] x86,word-at-a-time: " Peter Zijlstra
2021-11-05 18:01 ` Josh Poimboeuf
2021-11-05 18:07 ` Peter Zijlstra
2021-11-08 16:47 ` Josh Poimboeuf
2021-11-08 18:29 ` Peter Zijlstra
2021-11-08 18:53 ` Nick Desaulniers
2021-11-09 8:23 ` Peter Zijlstra
2021-11-09 19:22 ` Nick Desaulniers
2021-11-09 20:59 ` Bill Wendling
2021-11-09 21:21 ` Peter Zijlstra
2021-11-09 21:25 ` Nick Desaulniers
2021-11-09 22:11 ` Peter Zijlstra
2021-11-09 22:15 ` Nick Desaulniers
2021-11-09 21:07 ` Peter Zijlstra
2021-11-10 10:18 ` Peter Zijlstra
2021-11-10 10:46 ` David Laight
2021-11-10 11:09 ` Peter Zijlstra
2021-11-10 12:20 ` David Laight [this message]
2021-11-12 1:50 ` Josh Poimboeuf
2021-11-12 9:33 ` Peter Zijlstra
2021-11-13 5:35 ` Josh Poimboeuf
2021-11-15 12:36 ` Miroslav Benes
2021-11-15 13:01 ` Joe Lawrence
2021-11-15 23:40 ` Josh Poimboeuf
2021-11-16 7:25 ` Miroslav Benes
2021-11-15 12:59 ` Miroslav Benes
2021-11-16 21:27 ` Josh Poimboeuf
2021-11-18 7:15 ` Miroslav Benes
2021-11-22 17:46 ` Petr Mladek
2021-11-24 17:42 ` Josh Poimboeuf
2021-11-25 8:18 ` Petr Mladek
2021-11-10 12:14 ` Segher Boessenkool
2021-11-05 17:10 ` [PATCH 21/22] x86: Remove .fixup section Peter Zijlstra
2021-11-05 17:10 ` [PATCH 22/22] objtool: Remove .fixup handling Peter Zijlstra
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