On Wed, Dec 15, 2021 at 06:01:55PM -0800, Si-Wei Liu 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? meaning guest not acknowledging > > > VERSION_1 would use the legacy interfaces captured in the spec section 7.4 > > > (regarding ring layout, native endianness, message framing, vq alignment of > > > 4096, 32bit feature, no features_ok bit in status, IO port interface i.e. > > > all the things) instead? Noted we don't yet have a set_device_features() > > > that allows the vdpa device to tell whether it is operating in transitional > > > or modern-only mode. For software virtio, all support for the legacy part in > > > a transitional model has been built up there already, however, it's not easy > > > for vDPA vendors to implement all the requirements for an all-or-nothing > > > legacy guest support (big endian guest for example). To these vendors, the > > > legacy support within a transitional model is more of feature to them and > > > it's best to leave some flexibility for them to implement partial support > > > for legacy. That in turn calls out the need for a vhost-user protocol > > > feature like negotiation API that can prohibit those unsupported guest > > > setups to as early as backend_init before launching the VM. > > Right. Of note is the fact that it's a spec bug which I > > hope yet to fix, though due to existing guest code the > > fix won't be complete. > I thought at one point you pointed out to me that the spec does allow config > space read before claiming features_ok, and only config write before > features_ok is prohibited. I haven't read up the full thread of Halil's > VERSION_1 for transitional big endian device yet, but what is the spec bug > you hope to fix? Allowing config space reads before features_ok seemed useful years ago but in practice is only causing bugs and complicating device design. > > > > > WRT ioctls, One thing we can do though is abuse set_features > > where it's called by QEMU early on with just the VERSION_1 > > bit set, to distinguish between legacy and modern > > interface. This before config space accesses and FEATURES_OK. > > > > Halil has been working on this, pls take a look and maybe help him out. > Interesting thread, am reading now and see how I may leverage or help there. > > > > > > > > I > > > > > > > checked with Eli and other Mellanox/NVDIA folks for hardware/firmware level > > > > > > > 0.95 support, it seems all the ingredient had been there already dated back > > > > > > > to the DPDK days. The only major thing limiting is in the vDPA software that > > > > > > > the current vdpa core has the assumption around VIRTIO_F_ACCESS_PLATFORM for > > > > > > > a few DMA setup ops, which is virtio 1.0 only. > > > > > > > > > > > > > > > > 2. suppose some form of legacy guest support needs to be there, how do we > > > > > > > > > deal with the bogus assumption below in vdpa_get_config() in the short term? > > > > > > > > > It looks one of the intuitive fix is to move the vdpa_set_features call out > > > > > > > > > of vdpa_get_config() to vdpa_set_config(). > > > > > > > > > > > > > > > > > > /* > > > > > > > > > * Config accesses aren't supposed to trigger before features are > > > > > > > > > set. > > > > > > > > > * If it does happen we assume a legacy guest. > > > > > > > > > */ > > > > > > > > > if (!vdev->features_valid) > > > > > > > > > vdpa_set_features(vdev, 0); > > > > > > > > > ops->get_config(vdev, offset, buf, len); > > > > > > > > > > > > > > > > > > I can post a patch to fix 2) if there's consensus already reached. > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > -Siwei > > > > > > > > I'm not sure how important it is to change that. > > > > > > > > In any case it only affects transitional devices, right? > > > > > > > > Legacy only should not care ... > > > > > > > Yes I'd like to distinguish legacy driver (suppose it is 0.95) against the > > > > > > > modern one in a transitional device model rather than being legacy only. > > > > > > > That way a v0.95 and v1.0 supporting vdpa parent can support both types of > > > > > > > guests without having to reconfigure. Or are you suggesting limit to legacy > > > > > > > only at the time of vdpa creation would simplify the implementation a lot? > > > > > > > > > > > > > > Thanks, > > > > > > > -Siwei > > > > > > I don't know for sure. Take a look at the work Halil was doing > > > > > > to try and support transitional devices with BE guests. > > > > > Hmmm, we can have those endianness ioctls defined but the initial QEMU > > > > > implementation can be started to support x86 guest/host with little > > > > > endian and weak memory ordering first. The real trick is to detect > > > > > legacy guest - I am not sure if it's feasible to shift all the legacy > > > > > detection work to QEMU, or the kernel has to be part of the detection > > > > > (e.g. the kick before DRIVER_OK thing we have to duplicate the tracking > > > > > effort in QEMU) as well. Let me take a further look and get back. > > > > Michael may think differently but I think doing this in Qemu is much easier. > > > I think the key is whether we position emulating legacy interfaces in QEMU > > > doing translation on top of a v1.0 modern-only device in the kernel, or we > > > allow vdpa core (or you can say vhost-vdpa) and vendor driver to support a > > > transitional model in the kernel that is able to work for both v0.95 and > > > v1.0 drivers, with some slight aid from QEMU for > > > detecting/emulation/shadowing (for e.g CVQ, I/O port relay). I guess for the > > > former we still rely on vendor for a performant data vqs implementation, > > > leaving the question to what it may end up eventually in the kernel is > > > effectively the latter). > > > > > > Thanks, > > > -Siwei > > > > My suggestion is post the kernel patches, and we can evaluate > > how much work they are. > Thanks for the feedback. I will take some read then get back, probably after > the winter break. Stay tuned. > > Thanks, > -Siwei > > > > > > > Thanks > > > > > > > > > > > > > > > > > Meanwhile, I'll check internally to see if a legacy only model would > > > > > work. Thanks. > > > > > > > > > > Thanks, > > > > > -Siwei > > > > > > > > > > > > > > > > > > > On 3/2/2021 2:53 AM, Jason Wang wrote: > > > > > > > > > > On 2021/3/2 5:47 下午, Michael S. Tsirkin wrote: > > > > > > > > > > > On Mon, Mar 01, 2021 at 11:56:50AM +0800, Jason Wang wrote: > > > > > > > > > > > > On 2021/3/1 5:34 上午, Michael S. Tsirkin wrote: > > > > > > > > > > > > > On Wed, Feb 24, 2021 at 10:24:41AM -0800, Si-Wei Liu wrote: > > > > > > > > > > > > > > > Detecting it isn't enough though, we will need a new ioctl to notify > > > > > > > > > > > > > > > the kernel that it's a legacy guest. Ugh :( > > > > > > > > > > > > > > Well, although I think adding an ioctl is doable, may I > > > > > > > > > > > > > > know what the use > > > > > > > > > > > > > > case there will be for kernel to leverage such info > > > > > > > > > > > > > > directly? Is there a > > > > > > > > > > > > > > case QEMU can't do with dedicate ioctls later if there's indeed > > > > > > > > > > > > > > differentiation (legacy v.s. modern) needed? > > > > > > > > > > > > > BTW a good API could be > > > > > > > > > > > > > > > > > > > > > > > > > > #define VHOST_SET_ENDIAN _IOW(VHOST_VIRTIO, ?, int) > > > > > > > > > > > > > #define VHOST_GET_ENDIAN _IOW(VHOST_VIRTIO, ?, int) > > > > > > > > > > > > > > > > > > > > > > > > > > we did it per vring but maybe that was a mistake ... > > > > > > > > > > > > Actually, I wonder whether it's good time to just not support > > > > > > > > > > > > legacy driver > > > > > > > > > > > > for vDPA. Consider: > > > > > > > > > > > > > > > > > > > > > > > > 1) It's definition is no-normative > > > > > > > > > > > > 2) A lot of budren of codes > > > > > > > > > > > > > > > > > > > > > > > > So qemu can still present the legacy device since the config > > > > > > > > > > > > space or other > > > > > > > > > > > > stuffs that is presented by vhost-vDPA is not expected to be > > > > > > > > > > > > accessed by > > > > > > > > > > > > guest directly. Qemu can do the endian conversion when necessary > > > > > > > > > > > > in this > > > > > > > > > > > > case? > > > > > > > > > > > > > > > > > > > > > > > > Thanks > > > > > > > > > > > > > > > > > > > > > > > Overall I would be fine with this approach but we need to avoid breaking > > > > > > > > > > > working userspace, qemu releases with vdpa support are out there and > > > > > > > > > > > seem to work for people. Any changes need to take that into account > > > > > > > > > > > and document compatibility concerns. > > > > > > > > > > Agree, let me check. > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > I note that any hardware > > > > > > > > > > > implementation is already broken for legacy except on platforms with > > > > > > > > > > > strong ordering which might be helpful in reducing the scope. > > > > > > > > > > Yes. > > > > > > > > > > > > > > > > > > > > Thanks > > > > > > > > > > > > > > > > > > > > _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
