On Thu, May 19, 2022 at 06:39:39PM +0000, Raphael Norwitz wrote: > 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?
It's nice to have a single target (e.g. vhost-user-blk in SPDK) that handles all disk I/O. On the other hand, QEMU would still have the IDE/SATA emulation and libblkio vhost-user-blk driver, so in the end it may not reduce the amount of code that you need to support. Stefan
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