On Tue, May 17, 2022 at 03:53:52PM +0200, Paolo Bonzini wrote: > On 5/16/22 19:38, Raphael Norwitz wrote: > > [1] Keep using the SCSI translation in QEMU but back vDisks with a > > vhost-user-scsi or vhost-user-blk backend device. > > [2] Implement SATA and IDE emulation with vfio-user (likely with an SPDK > > client?). > > [3] We've also been looking at your libblkio library. From your > > description in > > https://urldefense.proofpoint.com/v2/url?u=https-3A__lists.gnu.org_archive_html_qemu-2Ddevel_2021-2D04_msg06146.html&d=DwICaQ&c=s883GpUCOChKOHiocYtGcg&r=In4gmR1pGzKB8G5p6LUrWqkSMec2L5EtXZow_FZNJZk&m=wBSqcw0cal3wPP87YIKgFgmqMHjGCC3apYf4wCn1SIrX6GW_FR-J9wO68v-cyrpn&s=CP-6ZY-gqgQ2zLAJdR8WVTrMBoqmFHilGvW_qnf2myU&e= > > it > > sounds like it may definitely play a role here, and possibly provide the > > nessesary abstractions to back I/O from these emulated disks to any > > backends we may want? > > First of all: have you benchmarked it? How much time is spent on MMIO vs. > disk I/O? >
Good point - we haven’t benchmarked the emulation, exit and translation overheads - it is very possible speeding up disk I/O may not have a huge impact. We would definitely benchmark this before exploring any of the options seriously, but as you rightly note, performance is not the only motivation here. > Of the options above, the most interesting to me is to implement a > vhost-user-blk/vhost-user-scsi backend in QEMU, similar to the NVMe one, > that would translate I/O submissions to virtqueue (including polling and the > like) and could be used with SATA. > We were certainly eyeing [1] as the most viable in the immediate future. That said, since a vhost-user-blk driver has been added to libblkio, [3] also sounds like a strong option. Do you see any long term benefit to translating SATA/IDE submissions to virtqueues in a world where libblkio is to be adopted? > For IDE specifically, I'm not sure how much it can be sped up since it has > only 1 in-flight operation. I think using KVM coalesced I/O could provide > an interesting boost (assuming instant or near-instant reply from the > backend). If all you're interested in however is not really performance, > but rather having a single "connection" to your back end, vhost-user is > certainly an option. > Interesting - I will take a look at KVM coalesced I/O. You’re totally right though, performance is not our main interest for these disk types. I should have emphasized offload rather than acceleration and performance. We would prefer to QA and support as few data paths as possible, and a vhost-user offload mechanism would allow us to use the same path for all I/O. I imagine other QEMU users who offload to backends like SPDK and use SATA/IDE disk types may feel similarly? > Paolo