Re: [PATCH] powerpc/64s: Add support for ASB_Notify on POWER9

[Michael Neuling <mikey@neuling.org>]

On Sat, 2017-08-05 at 14:28 +1000, Benjamin Herrenschmidt wrote:
> On Fri, 2017-08-04 at 16:56 +0200, Christophe Lombard wrote:
> > The POWER9 core supports a new feature: ASB_Notify which requires the
> > support of the Special Purpose Register: TIDR.
> >=20
> > The ASB_Notify command, generated by the AFU, will attempt to
> > wake-up the host thread identified by the particular LPID:PID:TID.
> >=20
> > The special register TIDR has to be updated to with the same value
> > present in the process element.
> >=20
> > If the length of the register TIDR is 64bits, the CAPI Translation
> > Service Layer core (XSL9) for Power9 systems limits the size (16bits) o=
f
> > the Thread ID when it generates the ASB_Notify message adding
> > PID:LPID:TID information from the context.
> >=20
> > The content of the internal kernel Thread ID (32bits) can not therefore
> > be used to fulfill the register TIDR.
> >=20
> > This patch allows to avoid this limitation by adding a new interface
> > for the user. The instructions mfspr/mtspr SPRN_TIDR are emulated,
> > save/restore SPRs (context switch) are updated and a new feature
> > (CPU_FTR_TIDR) is added to POWER9 system.
>=20
> Those CPU_FTR_* are internal to the kernel. You probably also need a
> feature in AT_HWCAP2 to indicate to userspace that this is supported.
>=20
> Also you put the onus of allocating the TIDs onto userspace which is a
> bit tricky. What happens if there are duplicate TIDs for example ? (ie,
> userspace doesn't allocate it or uses a library that spawns a thread)

I tend to agree.  I don't want userspace knowing anything about TIDR
allocations.  If we want userspace to receive one of these as_notifys, ther=
e
should be some abstract handle (like a file descriptor) that the kernel giv=
es
out. That handle should be associated with an LPID/PID/TID tuple by the ker=
nel.

This is similar to the PE number in CAPI. Most of the time userspace doesn'=
t
need to know its PE number (there is some cases were a master needs to know=
 a
slave PE, but that's the exception, not the rule).  Similarly, guests don't=
 know
their LPID.  Processes don't know their PID. Threads shouldn't know the TID=
.

Also, Suka has posted a patch that does TID allocation in the kernel...=C2=
=A0
http://patchwork.ozlabs.org/patch/799494/

Regards,
Mikey