Il 28/05/2013 20:15, Michael S. Tsirkin ha scritto: > What you write in spec below and what you write above seems to > contradict. > > Look, how about the simpler patch that I sent: > "[PATCH] virtio-spec: balloon: MUST_TELL_HOST is optional" > > does it functionally do everything you want in this patch? No, though it is a step forward. But let's take a step back instead; here are the requirements: 1) old drivers work unmodified on new hosts 2) new drivers work unmodified on old guests, though it is okay if they do not use silent deflation 3) if both the balloon and the driver supports silent deflation, it should be used 4) if the balloon has to enter a "degraded" mode of operation when guests do not "tell first", this has to happen only for old guests Once SILENT_DEFLATE is added, your patch prevents fulfilling requirements (3) and (4) together, at least in a Linux driver. Basically, you're saying that the driver should set MUST_TELL_HOST to !SILENT_DEFLATE. However, in the Linux virtio implementation, features are independent, and the feature list is told beforehand by the driver to the virtio layer; with your proposal, the driver would have to "retract" support for MUST_TELL_HOST after it has negotiated it. This is why I'm making SILENT_DEFLATE=1 override MUST_TELL_HOST=1. > If yes maybe you could pick that up, and add > a patch with renames and text clarifications on top? > > More comments below. > >> --- >> virtio-spec.lyx | 62 ++++++++++++++++++++++++++++++++++++++++++++++++++++----- >> 1 file changed, 57 insertions(+), 5 deletions(-) >> >> diff --git a/virtio-spec.lyx b/virtio-spec.lyx >> index adec0a5..5c76a87 100644 >> --- a/virtio-spec.lyx >> +++ b/virtio-spec.lyx >> @@ -7219,11 +7219,46 @@ bits >> >> \begin_deeper >> \begin_layout Description >> -VIRTIO_BALLOON_F_MUST_TELL_HOST >> +VIRTIO_BALLOON_F_ >> +\change_deleted 1531152142 1347020601 >> +MUST >> +\change_inserted 1531152142 1347020602 >> +CAN >> +\change_unchanged >> +_TELL_HOST >> \begin_inset space ~ >> \end_inset >> >> -(0) Host must be told before pages from the balloon are used. >> +(0) >> +\change_deleted 1531152142 1347020625 >> +Host must be told >> +\change_inserted 1531152142 1347020617 >> +Guest is able to tell host >> +\change_unchanged >> + before pages from the balloon are used. >> + > > This "can" and "is able" is IMO more confusing than clarifying. > Let's be definite. If feature is negotiated, guest must tell host. > If it's not, it does not. > > That's why it's MUST not CAN. No, that won't fly. First of all, it's not how other features work. A guest that asks for offloading can still do all the segmentation and checksumming itself, a guest that knows about the virtio-net control queue can still not use it, etc. Second, this prevents fulfilling requirements (3) and (4) together as I wrote above. > And text should be > > "When negotiated, guest tells host > before pages from the balloon are used. > " > > >> +\change_inserted 1531152142 1368005603 >> + The host must propose this feature > > We don't say "propose feature" anyway in the spec. > You really mean "set this feature bit" ? Yes. >> if it has to be told >> + before pages from the balloon are used. > > Has to? Not at all. It can't assume anything > until guest has negotiated the feature. > So it should be > "if it wants to be told before pages from the balloon are used". There can be implementations where the only choice is providing a fake balloon. For example if the page is removed from the IOMMU page tables, you have to tell the host before a DMA access. In this case, saying "has to be told" is not entirely precise, but "wants to be told" does not say that the operation is degraded. Any improvements on the wording are welcome. In the previous version I just said that all hosts should set this feature bit (and commented that old hosts can still be upwards-compatible). I changed it because you didn't like it, but I think the change is for the worse. I still really prefer the version I sent on May 8th. >> +\begin_inset Foot >> +status open >> + >> +\begin_layout Plain Layout >> + >> +\change_inserted 1531152142 1347022389 >> +This feature used to be named VIRTIO_BALLOON_F_\SpecialChar \- >> +MUST_TELL_HOST. >> + However, after a few years it was observed that drivers were not using >> + it as specified. >> + The virtio-balloon spec was then adjusted to what the drivers had been >> + doing. > > I'm not sure what good does this historical note do us. > I would say: > Historically the spec required all drivers > to acknowledge this feature bit. > However, no known hypervisor relies on this > feature bit being acknowledged unconditionally. > Thus, it's safe for drivers to ignore the TELL_HOST > feature. Ok. > > >> +\end_layout >> + >> +\end_inset >> + >> + >> +\change_unchanged >> + >> \end_layout >> >> \begin_layout Description >> @@ -7382,9 +7417,15 @@ The driver constructs an array of addresses of memory pages it has previously >> \end_layout >> >> \begin_layout Enumerate >> -If the VIRTIO_BALLOON_F_MUST_TELL_HOST feature is set, the guest may not >> - use these requested pages until that descriptor in the deflateq has been >> - used by the device. >> +If the VIRTIO_BALLOON_F_ >> +\change_deleted 1531152142 1369761770 >> +MUST >> +\change_inserted 1531152142 1369761770 >> +CAN >> +\change_unchanged >> +_TELL_HOST feature is set, the guest may not use these requested pages until >> + that descriptor in the deflateq has been used by the device. > > Not if it is set. If it's *negotiated*. Right. >> + >> \end_layout >> >> \begin_layout Enumerate >> @@ -7396,10 +7437,21 @@ status open >> >> \begin_layout Plain Layout >> In this case, deflation advice is merely a courtesy >> +\change_inserted 1531152142 1369761798 >> +. >> + The guest need not use the deflateq at all. >> +\change_unchanged >> + >> \end_layout >> >> \end_inset >> >> + >> +\change_inserted 1531152142 1369761801 >> + If the host does not support this, it should not do anything when the balloon >> + is inflated or deflated, except put the descriptors on the used ring. > > Not true. It simply can't rely on balloon to tell it > before pages are used. But, just as an example, > we can make kvm exit to qemu when guest > attempts to use such a page, and then we'll > know it is used without an explicit notification. Nice, but this is not always possible (I gave an example above). Paolo >> + >> +\change_unchanged >> >> \end_layout >> >> -- >> 1.8.2.1 >> -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
