Hi,
On 26.04.2016 16:23, Chris Wilson wrote:
On Tue, Apr 26, 2016 at 04:17:55PM +0300, Imre Deak wrote:
On ti, 2016-04-26 at 13:57 +0100, Chris Wilson wrote:
On Tue, Apr 26, 2016 at 03:44:22PM +0300, Imre Deak wrote:
Setting a write-back cache policy in the MOCS entry definition also
implies snooping, which has a considerable overhead. This is
unexpected for a few reasons:
If it is snooping, then I don't see why it is undesirable to have it
available in a mocs setting. If it is bogus and the bit is undefined,
then by all means remove it.
None of these entries are used alone for coherent surfaces. For that
the application would have to use entry index#1 or #2 _and_ call the
set caching IOCTL to set the corresponding buffer to be cached.
No, the application doesn't. There are sufficent interfaces exposed that
userspace can bypass the kernel if it so desired.
The
problem is that without setting the buffer to be cacheable the
expectation is that we won't be snooping and incur the corresponding
overhead. This is what this patch addresses.
Not true.
The bit is also bogus, if we wanted snooping via MOCS we'd use the
dedicated HW flag for that.
But you keep saying this bit *enables* snooping. So either it does or it
doesn't.
If we wanted to have a snooping MOCS entry we should add that
separately (as a forth entry), but we'd need this change as a fix for
current users.
The current users who are getting what they request but don't know what
they were requesting?
What this kernel ABI (index entry #2) has been agreed & documented to
provide?
I thought this entry is supposed to replace the writeback LLC/eLLC cache
MOCS setting Mesa is using on (e.g. BDW) to speed up accesses to a
memory area which it knows always to be accessed so that it can be cached.
If app runs on HW where LLC/eLLC is missing, giving the app extra
slowdown instead of potential speedup sounds like failed HW abstraction. :-)
- Eero
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