Memory barriers and spin_unlock safety

Paul Mackerras paulus at
Wed Mar 8 14:20:57 EST 2006

Linus Torvalds writes:

> So the rules from the PC side (and like it or not, they end up being 
> what all the drivers are tested with) are:
>  - regular stores are ordered by write barriers

I thought regular stores were always ordered anyway?

>  - PIO stores are always synchronous

By synchronous, do you mean ordered with respect to all other accesses
(regular memory, MMIO, prefetchable MMIO, PIO)?

In other words, if I store a value in regular memory, then do an
outb() to a device, and the device does a DMA read to the location I
just stored to, is the device guaranteed to see the value I just
stored (assuming no other later store to the location)?

>  - MMIO stores are ordered by IO semantics
> 	- PCI ordering must be honored:
> 	  * write combining is only allowed on PCI memory resources
> 	    that are marked prefetchable. If your host bridge does write 
> 	    combining in general, it's not a "host bridge", it's a "host 
> 	    disaster".

Presumably the host bridge doesn't know what sort of PCI resource is
mapped at a given address, so that information (whether the resource
is prefetchable) must come from the CPU, which would get it from the
TLB entry or an MTRR entry - is that right?

Or is there some gentleman's agreement between the host bridge and the
BIOS that certain address ranges are only used for certain types of
PCI memory resources?

> 	  * for others, writes can always be posted, but they cannot
> 	    be re-ordered wrt either reads or writes to that device
> 	    (ie a read will always be fully synchronizing)
> 	- io_wmb must be honored

What ordering is there between stores to regular memory and stores to
non-prefetchable MMIO?

If a store to regular memory can be performed before a store to MMIO,
does a wmb() suffice to enforce an ordering, or do you have to use

Do PCs ever use write-through caching on prefetchable MMIO resources?


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