Re: [PATCH 8/9] RISC-V: User-facing API

From: Will Deacon
Date: Tue Jul 11 2017 - 09:22:24 EST

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