Re: [RFC PATCH v4 18/20] vhost: Add VhostIOVATree

[Jason Wang <jasowang@redhat.com>]

On Thu, Oct 21, 2021 at 3:03 PM Eugenio Perez Martin
<eperezma@redhat.com> wrote:
>
> On Thu, Oct 21, 2021 at 4:34 AM Jason Wang <jasowang@redhat.com> wrote:
> >
> > On Wed, Oct 20, 2021 at 8:07 PM Eugenio Perez Martin
> > <eperezma@redhat.com> wrote:
> > >
> > > On Wed, Oct 20, 2021 at 11:01 AM Jason Wang <jasowang@redhat.com> wrote:
> > > >
> > > > On Wed, Oct 20, 2021 at 3:54 PM Eugenio Perez Martin
> > > > <eperezma@redhat.com> wrote:
> > > > >
> > > > > On Tue, Oct 19, 2021 at 11:23 AM Jason Wang <jasowang@redhat.com> wrote:
> > > > > >
> > > > > > On Tue, Oct 19, 2021 at 4:32 PM Jason Wang <jasowang@redhat.com> wrote:
> > > > > > >
> > > > > > >
> > > > > > > 在 2021/10/1 下午3:06, Eugenio Pérez 写道:
> > > > > > > > This tree is able to look for a translated address from an IOVA address.
> > > > > > > >
> > > > > > > > At first glance is similar to util/iova-tree. However, SVQ working on
> > > > > > > > devices with limited IOVA space need more capabilities, like allocating
> > > > > > > > IOVA chunks or perform reverse translations (qemu addresses to iova).
> > > > > > >
> > > > > > >
> > > > > > > I don't see any reverse translation is used in the shadow code. Or
> > > > > > > anything I missed?
> > > > > >
> > > > > > Ok, it looks to me that it is used in the iova allocator. But I think
> > > > > > it's better to decouple it to an independent allocator instead of
> > > > > > vhost iova tree.
> > > > > >
> > > > >
> > > > > Reverse translation is used every time a buffer is made available,
> > > > > since buffers content are not copied, only the descriptors to SVQ
> > > > > vring.
> > > >
> > > > I may miss something but I didn't see the code? Qemu knows the VA of
> > > > virtqueue, and the VA of the VQ is stored in the VirtQueueElem?
> > > >
> > >
> > > It's used in the patch 20/20, could that be the misunderstanding? The
> > > function calling it is vhost_svq_translate_addr.
> > >
> > > Qemu knows the VA address of the buffer, but it must offer a valid SVQ
> > > iova to the device. That is the translation I mean.
> >
> > Ok, I get you. So if I understand correctly, what you did is:
> >
> > 1) allocate IOVA during region_add
> > 2) preform VA->IOVA reverse lookup in handle_kick
> >
> > This should be fine, but here're some suggestions:
> >
> > 1) remove the assert(map) in vhost_svq_translate_addr() since guest
> > can add e.g BAR address
>
> Wouldn't VirtQueue block them in virtqueue_pop / address_space_read_*
> functions? I'm fine to remove it but I would say it adds value against
> coding error.

I think not. Though these addresses were excluded in
vhost_vdpa_listener_skipped_section(). For Qemu memory core, they are
valid addresses. Qemu emulate how hardware work (e.g pci p2p), so dma
to bar is allowed.

>
> > 2) we probably need a better name vhost_iova_tree_alloc(), maybe
> > "vhost_iova_tree_map_alloc()"
> >
>
> Ok I will change for the next version.
>
> > There's actually another method.
> >
> > 1) don't do IOVA/map allocation in region_add()
> > 2) do the allocation in handle_kick(), then we know the IOVA so no
> > reverse lookup
> >
> > The advantage is that this can work for the case of vIOMMU. And they
> > should perform the same:
> >
> > 1) you method avoid the iova allocation per sg
> > 2) my method avoid the reverse lookup per sg
> >
>
> It's somehow doable, but we are replacing a tree search with a linear
> insertion at this moment.
>
> I would say that guest's IOVA -> qemu vaddr part works with no change
> for vIOMMU, since VirtQueue's virtqueue_pop already gives us the vaddr
> even in the case of vIOMMU.

So in this case:

1) listener gives us GPA->host IOVA (host IOVA is allocated per GPA)
2) virtqueue_pop gives us guest IOVA -> VA

We still need extra logic to lookup the vIOMMU to get the guest IOVA
-> GPA then we can know the host IOVA.

If we allocate after virtqueue_pop(), we can follow the same logic as
without vIOMMU. Just allocate an host IOVA then all is done.

> The only change I would add for that case
> is the SVQ -> device map/unmapping part, so the device cannot access
> random addresses but only the exposed ones. I'm assuming that part is
> O(1).
>
> This way, we already have a tree with all the possible guest's
> addresses, and we only need to look for it's SVQ iova -> vaddr
> translation. This is a O(log(N)) operation,

Yes, but it's requires traverse the vIOMMU page table which should be
slower than our own iova tree?

> and read only, so it's
> easily parallelizable when we make each SVQ in it's own thread (if
> needed).

Yes, this is because the host IOVA was allocated before by the memory listener.

> The only thing left is to expose that with an iommu miss
> (O(1)) and unmap it on used buffers processing (also O(1)). The
> domination operation keeps being VirtQueue's own code lookup for
> guest's IOVA -> GPA, which I'm assuming is already well optimized and
> will benefit from future optimizations since qemu's memory system is
> frequently used.
>
> To optimize your use case we would need to add a custom (and smarter
> than the currently used) allocator to SVQ. I've been looking for ways
> to reuse glibc or similar in our own arenas but with no luck. It will
> be code that SVQ needs to maintain by and for itself anyway.

The benefit is to have separate iova allocation from the tree.

>
> In either case it should not be hard to switch to your method, just a
> few call changes in the future, if we achieve a faster allocator.
>
> Would that make sense?

Yes, feel free to choose any method you wish or feel simpler in the next series.

>
> > >
> > > > >
> > > > > At this point all the limits are copied to vhost iova tree in the next
> > > > > revision I will send, defined at its creation at
> > > > > vhost_iova_tree_new(). They are outside of util/iova-tree, only sent
> > > > > to the latter at allocation time.
> > > > >
> > > > > Since vhost_iova_tree has its own vhost_iova_tree_alloc(), that wraps
> > > > > the iova_tree_alloc() [1], limits could be kept in vhost-vdpa and make
> > > > > them an argument of vhost_iova_tree_alloc. But I'm not sure if it's
> > > > > what you are proposing or I'm missing something.
> > > >
> > > > If the reverse translation is only used in iova allocation, I meant to
> > > > split the logic of IOVA allocation itself.
> > > >
> > >
> > > Still don't understand it, sorry :). In SVQ setup we allocate an iova
> > > address for every guest's GPA address its driver can use. After that
> > > there should be no allocation unless memory is hotplugged.
> > >
> > > So the limits are only needed precisely at allocation time. Not sure
> > > if that is what you mean here, but to first allocate and then check if
> > > it is within the range could lead to false negatives, since there
> > > could be a valid range *in* the address but the iova allocator
> > > returned us another range that fell outside the range. How could we
> > > know the cause if it is not using the range itself?
> >
> > See my above reply. And we can teach the iova allocator to return the
> > IOVA in the range that vhost-vDPA supports.
> >
>
> Ok,
>
> For the next series it will be that way. I'm pretty sure we are
> aligned in this part, but the lack of code in this series makes it
> very hard to discuss it :).

Fine. Let's see.

Thanks

>
> Thanks!
>
> > Thanks
> >
> > >
> > > > >
> > > > > Either way, I think it is harder to talk about this specific case
> > > > > without code, since this one still does not address the limits. Would
> > > > > you prefer me to send another RFC in WIP quality, with *not* all
> > > > > comments addressed? I would say that there is not a lot of pending
> > > > > work to send the next one, but it might be easier for all of us.
> > > >
> > > > I'd prefer to try to address them all, otherwise it's not easy to see
> > > > what is missing.
> > > >
> > >
> > > Got it, I will do it that way then!
> > >
> > > Thanks!
> > >
> > > > Thanks
> > > >
> > > > >
> > > > > Thanks!
> > > > >
> > > > > [1] This util/iova-tree method will be proposed in the next series,
> > > > > and vhost_iova_tree wraps it since it needs to keep in sync both
> > > > > trees: iova->qemu vaddr for iova allocation and the reverse one to
> > > > > translate available buffers.
> > > > >
> > > > > > Thanks
> > > > > >
> > > > >
> > > >
> > >
> >
>