Messages in this thread | | | Date | Fri, 4 Aug 2023 21:16:20 +0200 | From | Andrea Parri <> | Subject | Re: [RFC PATCH] membarrier: riscv: Provide core serializing command |
| |
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 > > https://github.com/riscv/riscv-isa-manual/releases/download/Ratified-IMAFDQC/riscv-spec-20191213.pdf > > 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.
Andrea
| |