Re: [RFC PATCH] membarrier: riscv: Provide core serializing command

From: Andrea Parri
Date: Fri Aug 04 2023 - 15:16:33 EST

On Fri, Aug 04, 2023 at 02:05:55PM -0400, Mathieu Desnoyers wrote:
> On 8/4/23 10:59, Andrea Parri wrote:
> > > What is the relationship between FENCE.I and instruction cache flush on
> > > RISC-V ?
> >
> > The exact nature of this relationship is implementation-dependent. From
> > commentary included in the ISA portion referred to in the changelog:
> >
> > A simple implementation can flush the local instruction cache and
> > the instruction pipeline when the FENCE.I is executed. A more
> > complex implementation might snoop the instruction (data) cache on
> > every data (instruction) cache miss, or use an inclusive unified
> > private L2 cache to invalidate lines from the primary instruction
> > cache when they are being written by a local store instruction. If
> > instruction and data caches are kept coherent in this way, or if
> > the memory system consists of only uncached RAMs, then just the
> > fetch pipeline needs to be flushed at a FENCE.I. [..]
> >
> > Mmh, does this help?
> Quoting
> Chapter 3 "“Zifencei” Instruction-Fetch Fence, Version 2.0"
> "First, it has been recognized that on some systems, FENCE.I will be expensive to implement
> and alternate mechanisms are being discussed in the memory model task group. In particular,
> for designs that have an incoherent instruction cache and an incoherent data cache, or where
> the instruction cache refill does not snoop a coherent data cache, both caches must be completely
> flushed when a FENCE.I instruction is encountered. This problem is exacerbated when there are
> multiple levels of I and D cache in front of a unified cache or outer memory system.
> Second, the instruction is not powerful enough to make available at user level in a Unix-like
> operating system environment. The FENCE.I only synchronizes the local hart, and the OS can
> reschedule the user hart to a different physical hart after the FENCE.I. This would require the
> OS to execute an additional FENCE.I as part of every context migration. For this reason, the
> standard Linux ABI has removed FENCE.I from user-level and now requires a system call to
> maintain instruction-fetch coherence, which allows the OS to minimize the number of FENCE.I
> executions required on current systems and provides forward-compatibility with future improved
> instruction-fetch coherence mechanisms.
> Future approaches to instruction-fetch coherence under discussion include providing more
> restricted versions of FENCE.I that only target a given address specified in rs1, and/or allowing
> software to use an ABI that relies on machine-mode cache-maintenance operations."
> I start to suspect that even the people working on the riscv memory model have noticed
> that letting a single instruction such as FENCE.I take care of both cache coherency
> *and* flush the instruction pipeline will be a performance bottleneck, because it
> can only clear the whole instruction cache.
> Other architectures are either cache-coherent, or have cache flushing which can be
> performed on a range of addresses. This is kept apart from whatever instruction
> flushes the instruction pipeline of the processor.
> By keeping instruction cache flushing separate from instruction pipeline flush, we can
> let membarrier (and context switches, including thread migration) only care about the
> instruction pipeline part, and leave instruction cache flush to either a dedicated
> system call, or to specialized instructions which are available from user-mode.
> Considering that FENCE.I is forced to invalidate the whole i-cache, I don't think you
> will get away with executing it from switch_mm without making performance go down the
> drain on cache incoherent implementations.
> In my opinion, what we would need from RISC-V for membarrier (and context switch) is a
> lightweight version of FENCE.I which only flushes the instruction pipeline of the local
> processor. This should ideally come with a way for architectures with incoherent caches
> to flush the relevant address ranges of the i-cache which are modified by a JIT. This
> i-cache flush would not be required to flush the instruction pipeline, as it is typical
> to batch invalidation of various address ranges together and issue a single instruction
> pipeline flush on each CPU at the end. The i-cache flush could either be done by new
> instructions available from user-space (similar to aarch64), or through privileged
> instructions available through system calls (similar to arm cacheflush).

Thanks for the remarks, Mathieu. I think it will be very helpful to
RISC-V architects (and memory model people) to have this context and
reasoning written down.