From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stefano Stabellini Subject: Re: [PATCH 01/34] Documentation/memory-barriers.txt: document __smb_mb() Date: Mon, 4 Jan 2016 11:08:19 +0000 Message-ID: References: <1451473761-30019-1-git-send-email-mst@redhat.com> <1451473761-30019-2-git-send-email-mst@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Return-path: Received: from smtp.citrix.com ([66.165.176.89]:65150 "EHLO SMTP.CITRIX.COM" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751455AbcADLIo (ORCPT ); Mon, 4 Jan 2016 06:08:44 -0500 In-Reply-To: <1451473761-30019-2-git-send-email-mst@redhat.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: "Michael S. Tsirkin" Cc: linux-kernel@vger.kernel.org, Peter Zijlstra , Arnd Bergmann , linux-arch@vger.kernel.org, Andrew Cooper , virtualization@lists.linux-foundation.org, Stefano Stabellini , Jonathan Corbet , linux-doc@vger.kernel.org On Wed, 30 Dec 2015, Michael S. Tsirkin wrote: > Signed-off-by: Michael S. Tsirkin > --- > Documentation/memory-barriers.txt | 33 ++++++++++++++++++++++++++++----- > 1 file changed, 28 insertions(+), 5 deletions(-) > > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt > index aef9487..a20f7ef 100644 > --- a/Documentation/memory-barriers.txt > +++ b/Documentation/memory-barriers.txt > @@ -1655,17 +1655,18 @@ macro is a good place to start looking. > SMP memory barriers are reduced to compiler barriers on uniprocessor compiled > systems because it is assumed that a CPU will appear to be self-consistent, > and will order overlapping accesses correctly with respect to itself. > +However, see the subsection on "Virtual Machine Guests" below. > > [!] Note that SMP memory barriers _must_ be used to control the ordering of > references to shared memory on SMP systems, though the use of locking instead > is sufficient. > > Mandatory barriers should not be used to control SMP effects, since mandatory > -barriers unnecessarily impose overhead on UP systems. They may, however, be > -used to control MMIO effects on accesses through relaxed memory I/O windows. > -These are required even on non-SMP systems as they affect the order in which > -memory operations appear to a device by prohibiting both the compiler and the > -CPU from reordering them. > +barriers impose unnecessary overhead on both SMP and UP systems. They may, > +however, be used to control MMIO effects on accesses through relaxed memory I/O > +windows. These barriers are required even on non-SMP systems as they affect > +the order in which memory operations appear to a device by prohibiting both the > +compiler and the CPU from reordering them. > > > There are some more advanced barrier functions: > @@ -2948,6 +2949,28 @@ The Alpha defines the Linux kernel's memory barrier model. > > See the subsection on "Cache Coherency" above. > > +VIRTUAL MACHINE GUESTS > +------------------- > + > +Guests running within virtual machines might be affected by > +SMP effects even if the guest itself is compiled within ^ without > +SMP support. > + > +This is an artifact of interfacing with an SMP host while > +running an UP kernel. > + > +Using mandatory barriers for this use-case would be possible > +but is often suboptimal. > + > +To handle this case optimally, low-level __smp_mb() etc macros are available. > +These have the same effect as smp_mb() etc when SMP is enabled, but generate > +identical code for SMP and non-SMP systems. For example, virtual machine guests > +should use __smp_mb() rather than smp_mb() when synchronizing against a > +(possibly SMP) host. > + > +These are equivalent to smp_mb() etc counterparts in all other respects, > +in particular, they do not control MMIO effects: to control > +MMIO effects, use mandatory barriers. > > ============ > EXAMPLE USES > -- > MST >