On Fri, Dec 17, 2021 at 10:01 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > On Fri, Dec 17, 2021 at 09:57:38AM +0800, Jason Wang wrote: > > On Fri, Dec 17, 2021 at 6:32 AM Si-Wei Liu <si-wei.liu@xxxxxxxxxx> wrote: > > > > > > > > > > > > On 12/15/2021 6:53 PM, Jason Wang wrote: > > > > On Thu, Dec 16, 2021 at 10:02 AM Si-Wei Liu <si-wei.liu@xxxxxxxxxx> wrote: > > > >> > > > >> > > > >> On 12/15/2021 1:33 PM, Michael S. Tsirkin wrote: > > > >>> On Wed, Dec 15, 2021 at 12:52:20PM -0800, Si-Wei Liu wrote: > > > >>>> On 12/14/2021 6:06 PM, Jason Wang wrote: > > > >>>>> On Wed, Dec 15, 2021 at 9:05 AM Si-Wei Liu <si-wei.liu@xxxxxxxxxx> wrote: > > > >>>>>> On 12/13/2021 9:06 PM, Michael S. Tsirkin wrote: > > > >>>>>>> On Mon, Dec 13, 2021 at 05:59:45PM -0800, Si-Wei Liu wrote: > > > >>>>>>>> On 12/12/2021 1:26 AM, Michael S. Tsirkin wrote: > > > >>>>>>>>> On Fri, Dec 10, 2021 at 05:44:15PM -0800, Si-Wei Liu wrote: > > > >>>>>>>>>> Sorry for reviving this ancient thread. I was kinda lost for the conclusion > > > >>>>>>>>>> it ended up with. I have the following questions, > > > >>>>>>>>>> > > > >>>>>>>>>> 1. legacy guest support: from the past conversations it doesn't seem the > > > >>>>>>>>>> support will be completely dropped from the table, is my understanding > > > >>>>>>>>>> correct? Actually we're interested in supporting virtio v0.95 guest for x86, > > > >>>>>>>>>> which is backed by the spec at > > > >>>>>>>>>> https://urldefense.com/v3/__https://ozlabs.org/*rusty/virtio-spec/virtio-0.9.5.pdf__;fg!!ACWV5N9M2RV99hQ!dTKmzJwwRsFM7BtSuTDu1cNly5n4XCotH0WYmidzGqHSXt40i7ZU43UcNg7GYxZg$ . Though I'm not sure > > > >>>>>>>>>> if there's request/need to support wilder legacy virtio versions earlier > > > >>>>>>>>>> beyond. > > > >>>>>>>>> I personally feel it's less work to add in kernel than try to > > > >>>>>>>>> work around it in userspace. Jason feels differently. > > > >>>>>>>>> Maybe post the patches and this will prove to Jason it's not > > > >>>>>>>>> too terrible? > > > >>>>>>>> I suppose if the vdpa vendor does support 0.95 in the datapath and ring > > > >>>>>>>> layout level and is limited to x86 only, there should be easy way out. > > > >>>>>>> Note a subtle difference: what matters is that guest, not host is x86. > > > >>>>>>> Matters for emulators which might reorder memory accesses. > > > >>>>>>> I guess this enforcement belongs in QEMU then? > > > >>>>>> Right, I mean to get started, the initial guest driver support and the > > > >>>>>> corresponding QEMU support for transitional vdpa backend can be limited > > > >>>>>> to x86 guest/host only. Since the config space is emulated in QEMU, I > > > >>>>>> suppose it's not hard to enforce in QEMU. > > > >>>>> It's more than just config space, most devices have headers before the buffer. > > > >>>> The ordering in datapath (data VQs) would have to rely on vendor's support. > > > >>>> Since ORDER_PLATFORM is pretty new (v1.1), I guess vdpa h/w vendor nowadays > > > >>>> can/should well support the case when ORDER_PLATFORM is not acked by the > > > >>>> driver (actually this feature is filtered out by the QEMU vhost-vdpa driver > > > >>>> today), even with v1.0 spec conforming and modern only vDPA device. The > > > >>>> control VQ is implemented in software in the kernel, which can be easily > > > >>>> accommodated/fixed when needed. > > > >>>> > > > >>>>>> QEMU can drive GET_LEGACY, > > > >>>>>> GET_ENDIAN et al ioctls in advance to get the capability from the > > > >>>>>> individual vendor driver. For that, we need another negotiation protocol > > > >>>>>> similar to vhost_user's protocol_features between the vdpa kernel and > > > >>>>>> QEMU, way before the guest driver is ever probed and its feature > > > >>>>>> negotiation kicks in. Not sure we need a GET_MEMORY_ORDER ioctl call > > > >>>>>> from the device, but we can assume weak ordering for legacy at this > > > >>>>>> point (x86 only)? > > > >>>>> I'm lost here, we have get_features() so: > > > >>>> I assume here you refer to get_device_features() that Eli just changed the > > > >>>> name. > > > >>>>> 1) VERSION_1 means the device uses LE if provided, otherwise natvie > > > >>>>> 2) ORDER_PLATFORM means device requires platform ordering > > > >>>>> > > > >>>>> Any reason for having a new API for this? > > > >>>> Are you going to enforce all vDPA hardware vendors to support the > > > >>>> transitional model for legacy guest? > > > > Do we really have other choices? > > > > > > > > I suspect the legacy device is never implemented by any vendor: > > > > > > > > 1) no virtio way to detect host endian > > > This is even true for transitional device that is conforming to the > > > spec, right? > > > > For hardware, yes. > > > > > The transport specific way to detect host endian is still > > > being discussed and the spec revision is not finalized yet so far as I > > > see. Why this suddenly becomes a requirement/blocker for h/w vendors to > > > implement the transitional model? > > > > It's not a sudden blocker, the problem has existed since day 0 if I > > was not wrong. That's why the problem looks a little bit complicated > > and why it would be much simpler if we stick to modern devices. > > > > > Even if the spec is out, this is > > > pretty new and I suspect not all vendor would follow right away. I hope > > > the software framework can be tolerant with h/w vendors not supporting > > > host endianess (BE specifically) or not detecting it if they would like > > > to support a transitional device for legacy. > > > > Well, if we know we don't want to support the BE host it would be fine. > > I think you guys mean guest not host here. Same for memory ordering etc. > What matters is whether guest has barriers etc. > Yes. 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