On 10/12/2012 9:04 AM, Robert Watson wrote:
On Fri, 12 Oct 2012, John Baldwin wrote:
I believe it was because bus reads weren't guaranteed to be atomic
on i386. don't know if that's still the case or a concern, but it
was an intentional omission.
True. If you are on a 32-bit system you can read the two 4 byte
values and then build a 64-bit value. For 64-bit platforms we
should offer bus_read_8() however.
I believe there is still no way to perform a 64-bit read on a i386
(or at least without messing with SSE instructions), but if you have
to read a 64-bit register, you are stuck with doing two 32-bit reads
and concatenating them. I figure we may as well provide an
implementation for those who have to do that as well as the
implementation for 64-bit.
I think the problem though is that the way you should glue those two
32-bit reads together is device dependent. I don't think you can
provide a completely device-neutral bus_read_8() on i386. We should
certainly have it on 64-bit platforms, but I think drivers that want
to work on 32-bit platforms need to explicitly merge the two words
themselves.
Indeed -- and on non-x86, where there are uncached direct map
segments, and TLB entries that disable caching, reading 2x 32-bit vs
1x 64-bit have quite different effects in terms of atomicity. Where
uncached I/Os are being used, those differences may affect semantics
significantly -- e.g., if your device has a 64-bit memory-mapped FIFO
or registers, 2x 32-bit gives you two halves of two different 64-bit
values, rather than two halves of the same value. As device drivers
depend on those atomicity semantics, we should (at the busspace level)
offer only the exactly expected semantics, rather than trying to patch
things up. If a device driver accessing 64-bit fields wants to
support doing it using two 32-bit reads, it can figure out how to
splice it together following bus_space_read_region_4().
I wouldn't make any default behaviour for bus_space_read_8 on i386, just
amd64. My assumption (which may be unjustified) is that by far the most
common implementations to read a 64-bit register on i386 would be to
read the lower 4 bytes first, followed by the upper 4 bytes (or vice
versa) and then stitch them together. I think we should provide helper
functions for these two cases, otherwise I fear our code base will be
littered with multiple independent implementations of this.
Some driver writer who wants to take advantage of these helper functions
would do something like
#ifdef i386
#define bus_space_read_8 bus_space_read_8_lower_first
#endif
otherwise, using bus_space_read_8 won't compile for i386 builds.
If these implementations won't work for their case, they are free to
write their own implementation or take whatever action is necessary.
I guess my question is, are these cases common enough that it is worth
helping developers by providing functions that do the double read and
shifts for them, or do we leave them to deal with it on their own at the
risk of possibly some duplicated code.
Thanks,
Carl
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