On Mon, 29 Nov 2021 18:28:43 +0000 Alex Bennée <alex.ben...@linaro.org> wrote:
> Ben Widawsky <ben.widaw...@intel.com> writes: > > > On 21-11-26 12:08:08, Alex Bennée wrote: > >> > >> Ben Widawsky <ben.widaw...@intel.com> writes: > >> > >> > On 21-11-19 02:29:51, Shreyas Shah wrote: > >> >> Hi Ben > >> >> > >> >> Are you planning to add the CXL2.0 switch inside QEMU or already added > >> >> in one of the version? > >> >> > >> > > >> > From me, there are no plans for QEMU anything until/unless upstream > >> > thinks it > >> > will merge the existing patches, or provide feedback as to what it would > >> > take to > >> > get them merged. If upstream doesn't see a point in these patches, then > >> > I really > >> > don't see much value in continuing to further them. Once hardware comes > >> > out, the > >> > value proposition is certainly less. > >> > >> I take it: > >> > >> Subject: [RFC PATCH v3 00/31] CXL 2.0 Support > >> Date: Mon, 1 Feb 2021 16:59:17 -0800 > >> Message-Id: <20210202005948.241655-1-ben.widaw...@intel.com> > >> > >> is the current state of the support? I saw there was a fair amount of > >> discussion on the thread so assumed there would be a v4 forthcoming at > >> some point. > > > > Hi Alex, > > > > There is a v4, however, we never really had a solid plan for the primary > > issue > > which was around handling CXL memory expander devices properly (both from an > > interleaving standpoint as well as having a device which hosts multiple > > memory > > capacities, persistent and volatile). I didn't feel it was worth sending a > > v4 > > unless someone could say > > > > 1. we will merge what's there and fix later, or > > 2. you must have a more perfect emulation in place, or > > 3. we want to see usages for a real guest > > I think 1. is acceptable if the community is happy there will be ongoing > development and it's not just a code dump. Given it will have a > MAINTAINERS entry I think that is demonstrated. My thought is also 1. There are a few hacks we need to clean out but nothing that should take too long. I'm sure it'll take a rev or two more. Right now for example, I've added support to arm-virt and maybe need to move that over to a different machine model... > > What's the current use case? Testing drivers before real HW comes out? > Will it still be useful after real HW comes out for people wanting to > debug things without HW? CXL is continuing to expand in scope and capabilities and I don't see that reducing any time soon (My guess is 3 years+ to just catch up with what is under discussion today). So I see two long term use cases: 1) Automated verification that we haven't broken things. I suspect no one person is going to have a test farm covering all the corner cases. So we'll need emulation + firmware + kernel based testing. 2) New feature prove out. We have already used it for some features that will appear in the next spec version. Obviously I can't say what or send that code out yet. Its very useful and the spec draft has changed in various ways a result. I can't commit others, but Huawei will be doing more of this going forwards. For that we need a stable base to which we add the new stuff once spec publication allows it. > > > > > I had hoped we could merge what was there mostly as is and fix it up as we > > go. > > It's useful in the state it is now, and as time goes on, we find more > > usecases > > for it in a VMM, and not just driver development. > > > >> > >> Adding new subsystems to QEMU does seem to be a pain point for new > >> contributors. Patches tend to fall through the cracks of existing > >> maintainers who spend most of their time looking at stuff that directly > >> touches their files. There is also a reluctance to merge large chunks of > >> functionality without an identified maintainer (and maybe reviewers) who > >> can be the contact point for new patches. So in short you need: > >> > >> - Maintainer Reviewed-by/Acked-by on patches that touch other sub-systems > >> > > > > This is the challenging one. I have Cc'd the relevant maintainers (hw/pci > > and > > hw/mem are the two) in the past, but I think there interest is lacking (and > > reasonably so, it is an entirely different subsystem). > > So the best approach to that is to leave a Cc: tag in the patch itself > on your next posting so we can see the maintainer did see it but didn't > contribute a review tag. This is also a good reason to keep Message-Id > tags in patches so we can go back to the original threads. > > So in my latest PR you'll see: > > Signed-off-by: Willian Rampazzo <willi...@redhat.com> > Reviewed-by: Beraldo Leal <bl...@redhat.com> > Message-Id: <20211122191124.31620-1-willi...@redhat.com> > Signed-off-by: Alex Bennée <alex.ben...@linaro.org> > Reviewed-by: Philippe Mathieu-Daudé <phi...@redhat.com> > Message-Id: <20211129140932.4115115-7-alex.ben...@linaro.org> > > which shows the Message-Id from Willian's original posting and the > latest Message-Id from my posting of the maintainer tree (I trim off my > old ones). > > >> - Reviewed-by tags on the new sub-system patches from anyone who > >> understands CXL > > > > I have/had those from Jonathan. > > > >> - Some* in-tree testing (so it doesn't quietly bitrot) > > > > We had this, but it's stale now. We can bring this back up. > > > >> - A patch adding the sub-system to MAINTAINERS with identified people > > > > That was there too. Since the original posting, I'd be happy to sign > > Jonathan up > > to this if he's willing. > > Sounds good to me. Sure that's fine with me. Ben, I'm assuming you are fine with being joint maintainer? > > >> * Some means at least ensuring qtest can instantiate the device and not > >> fall over. Obviously more testing is better but it can always be > >> expanded on in later series. > > > > This was in the patch series. It could use more testing for sure, but I had > > basic functional testing in place via qtest. > > More is always better but the basic qtest does ensure a device doesn't > segfault if it's instantiated. I'll confess this is a bit I haven't looked at yet. Will get Shameer to give me a hand. Thanks Jonathan > > > > >> > >> Is that the feedback you were looking for? > > > > You validated my assumptions as to what's needed, but your first bullet is > > the > > one I can't seem to pin down. > > > > Thanks. > > Ben > >