Re: [PATCH 01/34] Documentation/memory-barriers.txt: document __smb_mb()

Stefano Stabellini
2016-01-04T11:08:47Z

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