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Date: Tue, 25 Aug 2020 10:22:38 -0700 From: Peter Oskolkov <posk@...k.io> To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com> Cc: linux-arch <linux-arch@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>, Peter Oskolkov <posk@...gle.com>, paulmck <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>, linux-kernel <linux-kernel@...r.kernel.org>, Paul Turner <pjt@...gle.com>, Chris Kennelly <ckennelly@...gle.com> Subject: Re: [PATCH 1/2 v3] rseq/membarrier: add MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ On Tue, Aug 25, 2020 at 9:58 AM Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote: > [...] > > Concretely speaking, let's just add a new membarrier command for the use-case > at hand. All other ways of doing things we have discussed are tricky to expose > in a way that is discoverable by user-space through the QUERY command. (using > a flag, or OR'ing many commands together) > > > > > 2: should @flags be repurposed for cpu_id, or MEMBARRIER_FLAG_CPU > > added with a new syscall parameter. > > => I'm still not sure a new parameter can be cleanly added, but I can try > > it in the next patchset if you prefer it this way. > > Yes please, it's easy to implement and we'll quickly see if anyone yells. If > it turns out to be a bad idea, you can always blame me. ;-) > > In summary: > > - We add 2 new membarrier commands: > - MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ > - MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ > > - We reserve a membarrier flag: > > enum membarrier_flag { > MEMBARRIER_FLAG_CPU = (1 << 0), > } > > So for CMD_PRIVATE_EXPEDITED_RSEQ, if flags & MEMBARRIER_FLAG_CPU is true, > then we expect the additional "int cpu" parameter (3rd parameter). Else the cpu > parameter is unused. > > Are you OK with this approach ? Yes, thanks for looking into this. I'll send a v4 later this week. Thanks, Peter [...]
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