Summary of LPC guest MSI discussion in Santa Fe (was: Re: [RFC 0/8] KVM PCIe/MSI passthrough on ARM/ARM64 (Alt II))

[Alex Williamson]

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