Re: [PATCH 0/3] VIRTIO-IOMMU: Introduce an aw-bits option

[Jean-Philippe Brucker]

On Mon, Jan 29, 2024 at 03:07:41PM +0100, Eric Auger wrote:
> Hi Jean-Philippe,
> 
> On 1/29/24 13:23, Jean-Philippe Brucker wrote:
> > Hi Eric,
> >
> > On Tue, Jan 23, 2024 at 07:15:54PM +0100, Eric Auger wrote:
> >> In [1] and [2] we attempted to fix a case where a VFIO-PCI device
> >> protected with a virtio-iommu is assigned to an x86 guest. On x86
> >> the physical IOMMU may have an address width (gaw) of 39 or 48 bits
> >> whereas the virtio-iommu exposes a 64b input address space by default.
> >> Hence the guest may try to use the full 64b space and DMA MAP
> >> failures may be encountered. To work around this issue we endeavoured
> >> to pass usable host IOVA regions (excluding the out of range space) from
> >> VFIO to the virtio-iommu device so that the virtio-iommu driver can
> >> query those latter during the probe request and let the guest iommu
> >> kernel subsystem carve them out.
> >>
> >> However if there are several devices in the same iommu group,
> >> only the reserved regions of the first one are taken into
> >> account by the iommu subsystem of the guest. This generally
> >> works on baremetal because devices are not going to
> >> expose different reserved regions. However in our case, this
> >> may prevent from taking into account the host iommu geometry.
> >>
> >> So the simplest solution to this problem looks to introduce an
> >> input address width option, aw-bits, which matches what is
> >> done on the intel-iommu. By default, from now on it is set
> >> to 39 bits with pc_q35 and 64b with arm virt.
> > Doesn't Arm have the same problem?  The TTB0 page tables limit what can be
> > mapped to 48-bit, or 52-bit when SMMU_IDR5.VAX==1 and granule is 64kB.
> > A Linux host driver could configure smaller VA sizes:
> > * SMMUv2 limits the VA to SMMU_IDR2.UBS (upstream bus size) which
> >   can go as low as 32-bit (I'm assuming we don't care about 32-bit hosts).
> Yes I think we can ignore that use case.
> > * SMMUv3 currently limits the VA to CONFIG_ARM64_VA_BITS, which
> >   could be as low as 36 bits (but realistically 39, since 36 depends on
> >   16kB pages and CONFIG_EXPERT).
> Further reading "3.4.1 Input address size and Virtual Address size" ooks
> indeed SMMU_IDR5.VAX gives info on the physical SMMU actual
> implementation max (which matches intel iommu gaw). I missed that. Now I
> am confused about should we limit VAS to 39 to accomodate of the worst
> case host SW configuration or shall we use 48 instead?

I don't know what's best either. 48 should be fine if hosts normally
enable VA_BITS_48 (I see debian has it [1], not sure how to find the
others).

[1] 
https://salsa.debian.org/kernel-team/linux/-/blob/master/debian/config/arm64/config?ref_type=heads#L18

> If we set such a low 39b value, won't it prevent some guests from
> properly working?

It's not that low, since it gives each endpoint a private 512GB address
space, but yes there might be special cases that reach the limit. Maybe
assign a multi-queue NIC to a 256-vCPU guest, and if you want per-vCPU DMA
pools, then with a 39-bit address space you only get 2GB per vCPU. With
48-bit you get 1TB which should be plenty.

52-bit private IOVA space doesn't seem useful, I doubt we'll ever need to
support that on the MAP/UNMAP interface.

So I guess 48-bit can be the default, and users with special setups can
override aw-bits.

Thanks,
Jean