linux-kernel - Re: [PATCH 01/34] Documentation/memory-barriers.txt: document __smb

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Message-ID: <alpine.DEB.2.02.1601041104290.19710@kaball.uk.xensource.com>
Date:	Mon, 4 Jan 2016 11:08:19 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	<linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnd Bergmann <arnd@...db.de>, <linux-arch@...r.kernel.org>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	<virtualization@...ts.linux-foundation.org>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Jonathan Corbet <corbet@....net>, <linux-doc@...r.kernel.org>
Subject: Re: [PATCH 01/34] Documentation/memory-barriers.txt: document
 __smb_mb()

On Wed, 30 Dec 2015, Michael S. Tsirkin wrote:
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
>  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
> 
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