On 05/10/2026 16:37, Boris Brezillon wrote: > On Mon, 5 Oct 2026 16:06:02 +0100 > Steven Price <[email protected]> wrote: > >> On 05/10/2026 09:38, Boris Brezillon wrote: >>> On Fri, 2 Oct 2026 16:14:38 +0100 >>> Steven Price <[email protected]> wrote: >>> >>>> On 29/09/2026 04:44, Adrián Larumbe wrote: >>>>> The GPU cache flush/invalidate operation is unnecessary. First off, the >>>>> GPU doesn't read off the perfcnt sample buffer, only writes into it, so >>>>> >>>> >>>> I don't think this is entirely true. The GPU performance counter unit >>>> only writes the counters that are enabled, counters that share a cache >>>> line but are not enabled are not written by the performance counter >>>> unit, but if the L2 contains that cache line then the write can hit in >>>> the L2 and dirty the entire line including stale data where the >>>> unwritten cache line is. >>> >>> I don't think this can happen though, because _enable_locked() is >>> creating a BO (and its GPU mapping) just before enabling the perfcnt >>> block, meaning the buffer is known to have no dirty cacheline pointing >>> to it until the first dump happens. And we do flush and invalidate GPU >>> caches after each dump, so again, we should be covered. >> >> The situation isn't actually a dirty cache line at the start, but a >> stale one. We start off with the memory matching a clean line in the >> GPU's cache. But because we don't have coherency the clean line can stay >> even if it's inconsistent with everything else. >> >> CPU | GPU | Memory >> ----------------+-----------------------+-------------------- >> | clean line | matches GPU cache >> ----------------+-----------------------+-------------------- >> CPU allocates new buffer and writes zeros >> ----------------+-----------------------+-------------------- >> Dirty cache line| stale clean line | unknown (cache line >> | | might be evicted) >> ----------------+-----------------------+-------------------- >> CPU cleans its own cache to memory >> ----------------+-----------------------+-------------------- >> Potential clean | stale clean line | Matches CPU >> cache line | | >> ----------------+-----------------------+-------------------- >> Start dump without invaliding GPU >> ----------------+-----------------------+-------------------- >> Potential clean | GPU writes data, and | Unknown >> cache line | hits in the clean line| >> | even though it's stale| >> ----------------+-----------------------+-------------------- >> CPU flushes the GPU's cache and invalidates it's own >> ----------------+-----------------------+-------------------- >> No-cache line | writes out clean line | Matches GPU >> >> >> Of course for the GPU to have ended up with that stale clean cache line >> means that the physical memory was previously used for something else on >> the GPU, so the newly allocated BO has to reuse memory from a previous >> BO that the GPU has accessed. And it's all "unlikely" due to the small >> size of the GPU's cache. > > Hm, I'd say it's actually impossible because every unmap operation is > followed by a flush+inval of the GPU L2 and LSC, so for this stale > clean line to exist on the GPU side when a physical page is GPU-mapped > again, it would take a bug in the unmap logic or in the MMU HW, I > think. Am I missing something?
Ah, yes that's true :) Although there's no need for us to do an invalidate on the unmap path... >> >> >>>> >>>> The upshot is that if the CPU has cleared a block of memory which the >>>> GPU happens to have cached, then the "unused" counters may end up >>>> showing the old data before the CPU cleared it (if they share a cache >>>> line with an active counter). >>> >>> I agree, but that's not a case we can hit in the enable path. I think I >>> mentioned the commit message was misleading, and that we should instead >>> talk about the fact the GPU is not supposed to have cached anything up >>> until the first SAMPLE following a the ENABLE step. >>> >>>> >>>> I have to admit it's probably somewhat academic given that Panfrost >>>> doesn't expose the ability to control which counters are enabled... >>>> >>>> Is there a good reason for this patch (i.e. have you seen a performance >>>> problem with doing the invalidate)? Otherwise I'd prefer we keep to the >>>> safe route rather than trying to over optimise cache maintenance. >>> >>> I think I was the one suggesting dropping this flush so that >>> panfrost_perfcnt_hw_enable() (in the last patch) has one less fallible >>> operation. Besides, I find it confusing to have a cache flush+inval in >>> a path where the GPU is not supposed to have accessed the buffer yet >>> (or later on, when we re-enable after a RESET, in a path where the GPU >>> has been reset and the caches are known to be empty). >> >> So I agree this Should Be Safe™ because of how the driver is currently >> using the performance counters. If we really want to drop the invalidate >> then I think we need a comment explaining the logic. My worry is that >> someone extends this in the future (e.g. allow selecting which counters >> to enable) and breaks assumptions without them being documented. >> >> We normally do perform an invalidate when the GPU first touches a buffer >> (e.g. for a BO) - it's just normally more implicit because it's done as >> part of the job manager(/command stream). >> >> Also "caches known to be empty" is a dangerous thing to assume on >> anything that could involve speculation - AFAIK Mali doesn't really >> perform any form of speculation, but I'm not 100% sure on that. >> Certainly with CPUs you don't get such a luxury of knowing what it might >> have populated in the caches. > > I suppose speculation pre-populating the caches with stale data would be > covered by the flush+inval we do after a map operation (this is > currently done in the unlock path regardless of the VM op, so both map > and unmap get it). I don't know, maybe the goal is to relax the flushing > policy around VM modifications in the future, but if things stay as > they are now, we can assume that a fresh BO being GPU-mapped guarantees > that no cacheline points to it until the first GPU access. But maybe > I'm missing something else... I have to admit I'm coming from experience on CPUs - there speculation means a CPU can load a cache line at basically any point. So the argument that a line cannot be in a cache almost never holds. GPUs (at least Mali GPUs) haven't quite got to the stage of CPUs. Thanks, Steve
