Re: [PATCH RFC LKMM 5/7] docs/memory-barriers.txt: Enforce heavy ordering for port I/O accesses

Peter Zijlstra
2019-01-11T09:53:50Z

Hi PeterA,

The Cover leter has this:

> 5. Update memory-barriers.txt on enforcing heavy ordering for
> port-I/O accesses, courtesy of Will Deacon. This one needs
> an ack, preferably by someone from Intel. Matthew Wilcox
> posted some feedback from an Intel manual here, which might
> be considered to be a close substitute, but... ;-)
>
> http://lkml.kernel.org/r/20181127192234.GF10377@bombadil.infradead.org

which in turn has:

> Here's a quote from Section 18.6 of volume 1 of the Software Developer
> Manual, November 2018 edition:
>
> When the I/O address space is used instead of memory-mapped I/O, the
> situation is different in two respects:
> • The processor never buffers I/O writes. Therefore, strict ordering of
> I/O operations is enforced by the processor. (As with memory-mapped I/O,
> it is possible for a chip set to post writes in certain I/O ranges.)
> • The processor synchronizes I/O instruction execution with external
> bus activity (see Table 18-1).
>
> Table 18-1 says that in* delays execution of the current instruction until
> completion of pending stores, and out* delays execution of the _next_
> instruction until completion of both pending stores and the current store.

Can we give an Intel ACK on the below patch?

On Wed, Jan 09, 2019 at 01:07:46PM -0800, Paul E. McKenney wrote:
> From: Will Deacon <will.deacon@arm.com>
>
> David Laight explains:
>
> | A long time ago there was a document from Intel that said that
> | inb/outb weren't necessarily synchronised wrt memory accesses.
> | (Might be P-pro era). However no processors actually behaved that
> | way and more recent docs say that inb/outb are fully ordered.
>
> This also reflects the situation on other architectures, the the port
> accessor macros tend to be implemented in terms of readX/writeX.
>
> Update Documentation/memory-barriers.txt to reflect reality.
>
> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: David Laight <David.Laight@ACULAB.COM>
> Cc: Alan Stern <stern@rowland.harvard.edu>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: <linux-arch@vger.kernel.org>
> Cc: <linux-doc@vger.kernel.org>
> Cc: <linux-kernel@vger.kernel.org>
> Signed-off-by: Will Deacon <will.deacon@arm.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
> ---
> Documentation/memory-barriers.txt | 6 ++----
> 1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index 1c22b21ae922..a70104e2a087 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -2619,10 +2619,8 @@ functions:
> intermediary bridges (such as the PCI host bridge) may not fully honour
> that.
>
> - They are guaranteed to be fully ordered with respect to each other.
> -
> - They are not guaranteed to be fully ordered with respect to other types of
> - memory and I/O operation.
> + They are guaranteed to be fully ordered with respect to each other and
> + also with respect to other types of memory and I/O operation.
>
> (*) readX(), writeX():
>
> --
> 2.17.1
>