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@xxxxxxxxxxxxxxxxxxxxxx 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@xxxxxxx> > > 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@xxxxxxxxxxxxxxxxxxx> > Cc: Arnd Bergmann <arnd@xxxxxxxx> > Cc: David Laight <David.Laight@xxxxxxxxxx> > Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > Cc: <linux-arch@xxxxxxxxxxxxxxx> > Cc: <linux-doc@xxxxxxxxxxxxxxx> > Cc: <linux-kernel@xxxxxxxxxxxxxxx> > Signed-off-by: Will Deacon <will.deacon@xxxxxxx> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx> > --- > 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 >
