Re: [RFC 0/7] VIRTIO-IOMMU/VFIO: Fix host iommu geometry handling for hotplugged devices

[Eric Auger <eric.auger@redhat.com>]

Hi Jean,

On 1/30/24 19:22, Jean-Philippe Brucker wrote:
> On Mon, Jan 29, 2024 at 05:38:55PM +0100, Eric Auger wrote:
>>> There may be a separate argument for clearing bypass. With a coldplugged
>>> VFIO device the flow is:
>>>
>>> 1. Map the whole guest address space in VFIO to implement boot-bypass.
>>>    This allocates all guest pages, which takes a while and is wasteful.
>>>    I've actually crashed a host that way, when spawning a guest with too
>>>    much RAM.
>> interesting
>>> 2. Start the VM
>>> 3. When the virtio-iommu driver attaches a (non-identity) domain to the
>>>    assigned endpoint, then unmap the whole address space in VFIO, and most
>>>    pages are given back to the host.
>>>
>>> We can't disable boot-bypass because the BIOS needs it. But instead the
>>> flow could be:
>>>
>>> 1. Start the VM, with only the virtual endpoints. Nothing to pin.
>>> 2. The virtio-iommu driver disables bypass during boot
>> We needed this boot-bypass mode for booting with virtio-blk-scsi
>> protected with virtio-iommu for instance.
>> That was needed because we don't have any virtio-iommu driver in edk2 as
>> opposed to intel iommu driver, right?
> Yes. What I had in mind is the x86 SeaBIOS which doesn't have any IOMMU
> driver and accesses the default SATA device:
>
>  $ qemu-system-x86_64 -M q35 -device virtio-iommu,boot-bypass=off
>  qemu: virtio_iommu_translate sid=250 is not known!!
>  qemu: no buffer available in event queue to report event
>  qemu: AHCI: Failed to start FIS receive engine: bad FIS receive buffer address
>
> But it's the same problem with edk2. Also a guest OS without a
> virtio-iommu driver needs boot-bypass. Once firmware boot is complete, the
> OS with a virtio-iommu driver normally can turn bypass off in the config
> space, it's not useful anymore. If it needs to put some endpoints in
> bypass, then it can attach them to a bypass domain.

yup
>
>>> 3. Hotplug the VFIO device. With bypass disabled there is no need to pin
>>>    the whole guest address space, unless the guest explicitly asks for an
>>>    identity domain.
>>>
>>> However, I don't know if this is a realistic scenario that will actually
>>> be used.
>>>
>>> By the way, do you have an easy way to reproduce the issue described here?
>>> I've had to enable iommu.forcedac=1 on the command-line, otherwise Linux
>>> just allocates 32-bit IOVAs.
>> I don't have a simple generic reproducer. It happens when assigning this
>> device:
>> Ethernet Controller E810-C for QSFP (Ethernet Network Adapter E810-C-Q2)
>>
>> I have not encountered that issue with another device yet.
>> I see on guest side in dmesg:
>> [    6.849292] ice 0000:00:05.0: Using 64-bit DMA addresses
>>
>> That's emitted in dma-iommu.c iommu_dma_alloc_iova().
>> Looks like the guest first tries to allocate an iova in the 32-bit AS
>> and if this fails use the whole dma_limit.
>> Seems the 32b IOVA alloc failed here ;-)
> Interesting, are you running some demanding workload and a lot of CPUs?
> That's a lot of IOVAs used up, I'm curious about what kind of DMA pattern
> does that.
Well nothing smart, just booting the guest with the assigned NIC. 8 vcpus

Thanks

Eric
>
> Thanks,
> Jean
>