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Re: [PATCH] vhost: Unbreak SMMU and virtio-iommu on dev-iotlb support

From: Peter Xu
Subject: Re: [PATCH] vhost: Unbreak SMMU and virtio-iommu on dev-iotlb support
Date: Sun, 7 Feb 2021 09:47:15 -0500

Hi, Kevin,

On Sun, Feb 07, 2021 at 09:04:55AM +0000, Tian, Kevin wrote:
> > From: Peter Xu
> > Sent: Friday, February 5, 2021 11:31 PM
> > 
> > > >
> > > >
> > > >> or virtio-iommu
> > > >> since dev-iotlb (or PCIe ATS)
> > > >
> > > >
> > > > We may need to add this in the future.
> > > added Jean-Philippe in CC
> > 
> > So that's the part I'm unsure about..  Since everybody is cced so maybe good
> > time to ask. :)
> > 
> > The thing is I'm still not clear on whether dev-iotlb is useful for a full
> > emulation environment and how that should differ from a normal iotlb, since
> > after all normal iotlb will be attached with device information too.
> dev-iotlb is useful in two manners. First, it's a functional prerequisite for
> supporting I/O page faults.

Is this also a hard requirement for virtio-iommu, which is not a real hardware
after all?

> Second, it has performance benefit as you don't
> need to contend the lock of global iotlb.

Hmm.. are you talking about e.g. vt-d driver or virtio-iommu?

Assuming it's about vt-d, qi_flush_dev_iotlb() will still call qi_submit_sync()
and taking the same global QI lock, as I see it, or I could be wrong somewhere.
I don't see where dev-iotlb has a standalone channel for delivery.

For virtio-iommu, we haven't defined dev-iotlb, right?  Sorry I missed things
when I completely didn't follow virtio-iommu recently - let's say if
virtio-iommu in the future can support per-dev dev-iotlb queue so it doesn't
need a global lock, what if we make it still per-device but still delivering
iotlb message?  Again, it's still a bit unclear to me why a full emulation
iommu would need that definition of "iotlb" and "dev-iotlb".

> > 
> > For real hardwares, they make sense because they ask for two things: iotlb 
> > is
> > for IOMMU, but dev-iotlb is for the device cache.  For emulation
> > environment
> > (virtio-iommu is the case) do we really need that complexity?
> > 
> > Note that even if there're assigned devices under virtio-iommu in the 
> > future,
> > we can still isolate that and iiuc we can easily convert an iotlb (from
> > virtio-iommu) into a hardware IOMMU dev-iotlb no matter what type of
> > IOMMU is
> > underneath the vIOMMU.
> > 
> Didn't get this point. Hardware dev-iotlb is updated by hardware (between
> the device and the IOMMU). How could software convert a virtual iotlb
> entry into hardware dev-iotlb?

I mean if virtio-iommu must be run in a guest, then we can trap that message
first, right?  If there're assigned device in the guest, we must convert that
invalidation to whatever message required for the host, that seems to not
require the virtio-iommu to have dev-iotlb knowledge, still?


Peter Xu

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