linux-kernel - Re: [PATCH] xen: Use wmb instead of rmb in xen_evtchn_do

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: <200806101815.53729.nickpiggin@yahoo.com.au>
Date:	Tue, 10 Jun 2008 18:15:53 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Isaku Yamahata <yamahata@...inux.co.jp>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-ia64-devel@...ts.xensource.com,
	Samuel Thibault <samuel.thibault@...citrix.com>,
	Keir Fraser <Keir.Fraser@...citrix.com>
Subject: Re: [PATCH] xen: Use wmb instead of rmb in xen_evtchn_do_upcall().

On Tuesday 10 June 2008 17:57, Jeremy Fitzhardinge wrote:
> Nick Piggin wrote:
> > On Tuesday 10 June 2008 17:35, Isaku Yamahata wrote:
> >> This patch is ported one from 534:77db69c38249 of linux-2.6.18-xen.hg.
> >> Use wmb instead of rmb to enforce ordering between
> >> evtchn_upcall_pending and evtchn_pending_sel stores
> >> in xen_evtchn_do_upcall().
> >
> > There are a whole load of places in the kernel that should be using
> > smp_ variants of memory barriers. This seemed to me like one of them,
> > but I could be wrong.
>
> No, it needs to be an unconditional barrier.  This is synchronizing with
> the hypervisor - even if the kernel is compiled UP, the SMP hypervisor
> may be testing/setting the events pending bits from another (physical) cpu.

OK. What you *really* want is smp_*mb_even_if_compiled_for_UP() ;)
That is, a small set of primitives that are compiled with CONFIG_SMP
(and given some xxx_ prefix to distinguish).

IO barriers are probably the best thing you can use for the moment.


> > Also, if you do that can you get rid of the ifdef? If it really *really*
> > mattered, we could introduce smp_mb before/after xchg... but if you
> > use smp_wmb anyway then it definitely does not matter because that is a
> > noop on x86.
>
> Yes, I'd like to lose the #ifdef.  Unfortunately I think putting a
> "lock; addl $0,0(%%esp)" style barrier had a measurable negative
> performance impact, but I may be thinking of something else.  I don't
> know how expensive sfence is.
>
> The alternative is to make ia64's xchg a barrier (or to add a barrier
> variant of it).  It seems like a wart to have a cross-architecture
> function like xchg(), but then have different architectures differ in
> important details like barrier-ness.

Well, no you have to be careful. Because even if we did ask for ia64's
xchg to be a full barrier, you wouldn't get the right behaviour on UP
because it would be free to optimise that away -- those kinds of barriers
referred to in all those primitives are defined for cacheable memory and
CPU-to-CPU only...

I guess under the circumstances, leaving the ifdef there is probably
reasonable.
--
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/