Cc: Andi Kleen <ak@...>, <linux-kernel@...>, Thomas Gleixner <tglx@...>, H. Peter Anvin <hpa@...>, Venki Pallipadi <venkatesh.pallipadi@...>, <suresh.b.siddha@...>, Arjan van de Ven <arjan@...>, Dave Jones <davej@...>
On Thu, Jan 10, 2008 at 11:57:26AM +0100, Ingo Molnar wrote:
Cached requires the cache line to be read first before you can write it.
WC on the other hand does not allocate a cache line and just dumps
the data into a special write combining buffer. It was invented originally
because reads from AGP were incredibly slow.
And it's race less regarding the caching protocol (assuming you flush
the caches and TLBs correctly).
Another typical problem is that if something
is uncached then you can't have it in any other caches because if that
cache eventually flushes it will corrupt the data.
That can happen with remapping apertures for example which remap data
behind the CPUs back.
CLFLUSH is really only a hint but it cannot be used if UC is needed
for correctness.
The typical case would be lots of user space DRI clients supplying
their own buffers on the fly. There's not really a fixed pool
in this case, but it all varies dynamically. In some scenarios
that could happen quite often.
-Andi
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