Re: [patch] SMP alternatives

[Andi Kleen <ak@suse.de>]

> 1. The lock behaviour *is* defined for main memory access by all bus
> masters.

For uncached memory, right?

> 2. Uncached mappings are unworkable for this because we must never have
> a page mapped with conflicting cache types - thats ugly, and plain
> horrific on SMP.

For kernel mapping change_page_attr() takes care of it,
and for user space memory following all mappings is the only
reliable way to find out which process needs to be killed
anyways - and when you do that you can as well unmap
or just kill.

> 3. Uncached has undefined semantics when racing a PCI master. Lock has
> defined semantics. An uncached add #0 is permitted to read the memory
> and then write it back as two different cycles and I suspect does.

Consider what happens with such a race: either the PCI master
gets an bus abort because it still sees the corrupted data.
Or it already accesses the repaired data. Both is ok.

> 4. The AMD BIOS guide requires both that LOCK is enabled by default and
> that the "lock affects the external bus" bit is clear to enable locking
> on the external bus.

The "Linux guidelines" might be different.

-Andi