On Tue, 11 Jul 2017 06:22:15 PDT (-0700), will.deacon@xxxxxxx wrote: > On Mon, Jul 10, 2017 at 01:00:29PM -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. > > Take a look at the ARM code I mentioned. You can do away with the syscall if > you notice that you preempt a thread inside the critical section of the > vDSO, and, in that case you resume execution at a known "restart" address. > >> >> 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. > > The problem is that by supporting these hypothetical designs that can't do > atomics, you hurt sensible designs that *can* do the atomics because you > force them to take an additional indirection that could otherwise be > avoided. I just went ahead and removed the system calls from the port -- they're not going to get called on any systems SiFive is building, so if someone complains then we'll just sort it out later.
