Messages in this thread | | | Date | Sat, 08 Jul 2006 13:18:16 +0300 | From | Avi Kivity <> | Subject | Re: [patch] spinlocks: remove 'volatile' |
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Arjan van de Ven wrote: > > > > > > with PCI, and the PCI posting rules, there is no "one" serializing > > > instruction, you need to know the specifics of the device in > question to > > > cause the flush. So at least there is no universal possible > > > implementation of volatile as you suggest ;-) > > > > > > > A serializing volatile makes it possible to write portable code to > > access pci mmio. You'd just follow a write with a read or whatever the > > rules say. > > yeah except that the compiler cannot know what to read; reading back the > same memory location is NOT correct nor safe. It's device specific, for > some devices it'll be safe, for others you have to read some OTHER > location. >
I didn't suggest the compiler could or should do it, just that it would be possible (for the _user_) to write portable ISO C code to access PCI mmio registers, if volatile's implementation serialized access.
With the current implementation of volatile in gcc, it is impossible - you need to resort to inline assembly for some architectures, which is not an ISO C feature.
And I'm not suggesting that it would be a good idea to use volatile even if it was corrected - it would have to take a worst-case approach and thus would generate very bad code.
-- Do not meddle in the internals of kernels, for they are subtle and quick to panic.
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