[dpdk-dev] [RFC 1/6] eal: add power management intrinsics

[Bruce Richardson]

On Thu, May 28, 2020 at 03:40:18PM +0100, Burakov, Anatoly wrote:
> On 28-May-20 12:39 PM, Ananyev, Konstantin wrote:
> > Hi Anatoly,
> > 
> > > 
> > > Add two new power management intrinsics, and provide an implementation
> > > in eal/x86 based on UMONITOR/UMWAIT instructions. The instructions
> > > are implemented as raw byte opcodes because there is not yet widespread
> > > compiler support for these instructions.
> > > 
> > > The power management instructions provide an architecture-specific
> > > function to either wait until a specified TSC timestamp is reached, or
> > > optionally wait until either a TSC timestamp is reached or a memory
> > > location is written to. The monitor function also provides an optional
> > > comparison, to avoid sleeping when the expected write has already
> > > happened, and no more writes are expected.
> > 
> > Recently ARM guys introduced new generic API
> > for similar (as I understand) purposes: rte_wait_until_equal_(16|32|64).
> > Probably would make sense to unite both APIs into something common
> > and HW transparent.
> > Konstantin
> 
> Hi Konstantin,
> 
> That's not really similar purpose. This is monitoring a cacheline for
> writes, not waiting on a specific value. The "expected" value is there as
> basically a hack to get around the race condition due to the fact that by
> the time you enter monitoring state, the write you're waiting for may have
> already happened.
> 
Rather than the "expected" value, is it not more useful for a general API
to check for the existing value? Since we are awaiting writes, we may not
know what new value will be written, but we do know what the value is now,
and can just check that it's not been changed.

Regards,
/Bruce