On Wed, Apr 09, 2025 at 02:24:32PM +0200, David Hildenbrand wrote: > On 09.04.25 14:07, Michael S. Tsirkin wrote: > > On Wed, Apr 09, 2025 at 01:12:19PM +0200, David Hildenbrand wrote: > > > On 09.04.25 12:56, Michael S. Tsirkin wrote: > > > > On Wed, Apr 09, 2025 at 12:46:41PM +0200, David Hildenbrand wrote: > > > > > On 07.04.25 23:20, Michael S. Tsirkin wrote: > > > > > > On Mon, Apr 07, 2025 at 08:47:05PM +0200, David Hildenbrand wrote: > > > > > > > > In my opinion, it makes the most sense to keep the spec as it > > > > > > > > is and > > > > > > > > change QEMU and the kernel to match, but obviously that's not > > > > > > > > trivial > > > > > > > > to do in a way that doesn't break existing devices and drivers. > > > > > > > > > > > > > > If only it would be limited to QEMU and Linux ... :) > > > > > > > > > > > > > > Out of curiosity, assuming we'd make the spec match the current > > > > > > > QEMU/Linux > > > > > > > implementation at least for the 3 involved features only, would > > > > > > > there be a > > > > > > > way to adjust crossvm without any disruption? > > > > > > > > > > > > > > I still have the feeling that it will be rather hard to get that > > > > > > > all > > > > > > > implementations match the spec ... For new features+queues it > > > > > > > will be easy > > > > > > > to force the usage of fixed virtqueue numbers, but for > > > > > > > free-page-hinting and > > > > > > > reporting, it's a mess :( > > > > > > > > > > > > > > > > > > Still thinking about a way to fix drivers... We can discuss this > > > > > > theoretically, maybe? > > > > > > > > > > Yes, absolutely. I took the time to do some more digging; regarding > > > > > drivers > > > > > only Linux seems to be problematic. > > > > > > > > > > virtio-win, FreeBSD, NetBSD and OpenBSD and don't seem to support > > > > > problematic features (free page hinting, free page reporting) in their > > > > > virtio-balloon implementations. > > > > > > > > > > So from the known drivers, only Linux is applicable. > > > > > > > > > > reporting_vq is either at idx 4/3/2 > > > > > free_page_vq is either at idx 3/2 > > > > > statsq is at idx2 (only relevant if the feature is offered) > > > > > > > > > > So if we could test for the existence of a virtqueue at an idx > > > > > easily, we > > > > > could test from highest-to-smallest idx. > > > > > > > > > > But I recall that testing for the existance of a virtqueue on s390x > > > > > resulted > > > > > in the problem/deadlock in the first place ... > > > > > > > > > > -- > > > > > Cheers, > > > > > > > > > > David / dhildenb > > > > > > > > So let's talk about a new feature bit? > > > > > > Are you thinking about a new feature that switches between "fixed queue > > > indices" and "compressed queue indices", whereby the latter would be the > > > legacy default and we would expect all devices to switch to the new > > > fixed-queue-indices layout? > > > > > > We could make all new features require "fixed-queue-indices". > > > > I see two ways: > > 1. we make driver behave correctly with in spec and out of spec devices > > and we make qemu behave correctly with in spec and out of spec devices > > 2. a new feature bit > > > > I prefer 1, and when we add a new feature we can also > > document that it should be in spec if negotiated. > > > > My question is if 1 is practical. > > AFAIKT, 1) implies: > > virtio-balloon: > > a) Driver > > As mentioned above, we'd need a reliable way to test for the existence of a > virtqueue, so we can e.g., test for reporting_vq idx 4 -> 3 -> 2 > > With that we'd be able to support compressed+fixed at the same time. > > Q: Is it possible/feasible? > > b) Device: virtio-balloon: we can fake existence of STAT and > FREE_PAGE_HINTING easily, such that the compressed layout corresponds to the > fixed layout easily. > > Q: alternatives? We could try creating multiple queues for the same feature, > but it's going to be a mess I'm afraid ... > > > virtio-fs: > > a) Driver > > Linux does not even implement VIRTIO_FS_F_NOTIFICATION or respect > VIRTIO_FS_F_NOTIFICATION when calculating queue indices, ... > > b) Device > > Same applies to virtiofsd ... > > Q: Did anybody actually implement VIRTIO_FS_F_NOTIFICATION ever? If not, can > we just remove it from the spec completely and resolve the issue that way?
Donnu. Vivek? Or we can check for queue number 1+num_request_queues maybe? If that exists then it is spec compliant? > -- > Cheers, > > David / dhildenb
