On Mon, 10 Jul 2017 13:00:29 PDT (-0700), Palmer Dabbelt wrote: > On Thu, 06 Jul 2017 08:45:13 PDT (-0700), will.deacon@xxxxxxx wrote: >> On Thu, Jul 06, 2017 at 08:34:27AM -0700, Christoph Hellwig wrote: >>> On Thu, Jul 06, 2017 at 09:55:03AM +0100, Will Deacon wrote: >>> > Agreed on the indirection; it feels like this is something that should be in >>> > the vDSO, which could use the cmpxchg instruction if it's available, or >>> > otherwise just uses plain loads and stores. > > These are already in the vDSO, and use the corresponding atomic instructions on > systems with the A extension. The vDSO routines call the system calls in non-A > systems. As far as I can tell that's necessary to preserve atomicity, which we > currently do by disabling scheduling. If there's a way to do this without > entering the kernel then I'd be happy to support it, but I'm not sure how we > could maintain atomicity using only regular loads and stores. > >>> Even that seems like a lot of indirection for something that is in >>> the critical fast path for synchronization. I really can't understand >>> how a new ISA / ABI could even come up with an idea as stupid as making >>> essential synchronization primitives optional. >> >> No disagreement there! > > The default set of multilibs on Linux are: > > * rv32imac: 32-bit; Multiply, Atomic, and Compressed extensions > * rv32imafdc: like above, but with single+double float > * rv64imac: 64-bit, Multiply, Atomic and Compressed > * rv64imafdc: like above, but with single+double float > > all of which support the A extension. We certainly don't plan on building any > systems that support Linux without the A extension at SiFive, so I'm fine > removing the system call -- this was originally added by a user, so there was > at least enough interest for someone to add the system call. > > We've found people are retrofitting other cores to run RISC-V, and I could > certainly imagine an older design that lacks a beefy enough memory system to > support our atomics (which are LR/SC based) being a design that might arise. > There's a lot of systems where people don't seem to care that much about the > performance and just want something to work -- if they're on such a tiny system > they can't implement the A extension then they're probably not going to be > doing a lot of atomics anyway, so maybe it doesn't matter if atomics are slow. > As the cost for supporting these A-less systems seems fairly small, it seemed > like the right thing to do -- one of the points of making RISC-V have many > optional extensions was to let people pick the ones they view as important. > Since I don't know the performance constraints of their systems or the cost of > implementing the A extension in their design, I'm not really qualified to tell > them a cmpxchg syscall is a bad idea. > > I'm fine either way here: if someone's core can't support the A extension they > can always just buy one that does (ideally from us :)). If it was up to be I'd > leave the calls in there, as I generally don't like to tell users we won't > support their use case, but since you guys seem to know a lot more about this > than I do I'll just leave the decision up to you. > > If you want the system call (and the corresponding vDSO entry, which will be > unnecessary if we mandate A) gone then I'll remove it for our v5. Just give me > a heads up. > > Thanks, and sorry for wasting your time! I mangled this message when sending it so I'm trying again.
