Summary of LPC guest MSI discussion in Santa Fe

[Don Dutile]

On 11/08/2016 06:35 PM, Alex Williamson wrote:
> On Tue, 8 Nov 2016 21:29:22 +0100
> Christoffer Dall <christoffer.dall at linaro.org> wrote:
>
>> Hi Will,
>>
>> On Tue, Nov 08, 2016 at 02:45:59AM +0000, Will Deacon wrote:
>>> Hi all,
>>>
>>> I figured this was a reasonable post to piggy-back on for the LPC minutes
>>> relating to guest MSIs on arm64.
>>>
>>> On Thu, Nov 03, 2016 at 10:02:05PM -0600, Alex Williamson wrote:
>>>> We can always have QEMU reject hot-adding the device if the reserved
>>>> region overlaps existing guest RAM, but I don't even really see how we
>>>> advise users to give them a reasonable chance of avoiding that
>>>> possibility.  Apparently there are also ARM platforms where MSI pages
>>>> cannot be remapped to support the previous programmable user/VM
>>>> address, is it even worthwhile to support those platforms?  Does that
>>>> decision influence whether user programmable MSI reserved regions are
>>>> really a second class citizen to fixed reserved regions?  I expect
>>>> we'll be talking about this tomorrow morning, but I certainly haven't
>>>> come up with any viable solutions to this.  Thanks,
>>>
>>> At LPC last week, we discussed guest MSIs on arm64 as part of the PCI
>>> microconference. I presented some slides to illustrate some of the issues
>>> we're trying to solve:
>>>
>>>    http://www.willdeacon.ukfsn.org/bitbucket/lpc-16/msi-in-guest-arm64.pdf
>>>
>>> Punit took some notes (thanks!) on the etherpad here:
>>>
>>>    https://etherpad.openstack.org/p/LPC2016_PCI
>>>
>>> although the discussion was pretty lively and jumped about, so I've had
>>> to go from memory where the notes didn't capture everything that was
>>> said.
>>>
>>> To summarise, arm64 platforms differ in their handling of MSIs when compared
>>> to x86:
>>>
>>>    1. The physical memory map is not standardised (Jon pointed out that
>>>       this is something that was realised late on)
>>>    2. MSIs are usually treated the same as DMA writes, in that they must be
>>>       mapped by the SMMU page tables so that they target a physical MSI
>>>       doorbell
>>>    3. On some platforms, MSIs bypass the SMMU entirely (e.g. due to an MSI
>>>       doorbell built into the PCI RC)
>>>    4. Platforms typically have some set of addresses that abort before
>>>       reaching the SMMU (e.g. because the PCI identifies them as P2P).
>>>
>>> All of this means that userspace (QEMU) needs to identify the memory
>>> regions corresponding to points (3) and (4) and ensure that they are
>>> not allocated in the guest physical (IPA) space. For platforms that can
>>> remap the MSI doorbell as in (2), then some space also needs to be
>>> allocated for that.
>>>
>>> Rather than treat these as separate problems, a better interface is to
>>> tell userspace about a set of reserved regions, and have this include
>>> the MSI doorbell, irrespective of whether or not it can be remapped.
>>
>> Is my understanding correct, that you need to tell userspace about the
>> location of the doorbell (in the IOVA space) in case (2), because even
>> though the configuration of the device is handled by the (host) kernel
>> through trapping of the BARs, we have to avoid the VFIO user programming
>> the device to create other DMA transactions to this particular address,
>> since that will obviously conflict and either not produce the desired
>> DMA transactions or result in unintended weird interrupts?
>
> Correct, if the MSI doorbell IOVA range overlaps RAM in the VM, then
> it's potentially a DMA target and we'll get bogus data on DMA read from
> the device, and lose data and potentially trigger spurious interrupts on
> DMA write from the device.  Thanks,
>
> Alex
>
That's b/c the MSI doorbells are not positioned *above* the SMMU, i.e.,
they address match before the SMMU checks are done.  if
all DMA addrs had to go through SMMU first, then the DMA access could
be ignored/rejected.
For bare-metal, memory can't be put in the same place as MSI addrs, or
DMA could never reach it.  So, only a virt issue, unless the VMs mem address
range mimic the host layout.

- Don