Re: [PATCH v3 2/2] spapr: Enable virtio iommu_platform=on by default

[Greg Kurz]

On Thu, 5 Mar 2020 12:59:03 +0100
Greg Kurz <address@hidden> wrote:

> On Thu,  5 Mar 2020 15:30:09 +1100
> David Gibson <address@hidden> wrote:
> 
> > Traditionally, virtio devices don't do DMA by the usual path on the
> > guest platform.  In particular they usually bypass any virtual IOMMU
> > the guest has, using hypervisor magic to access untranslated guest
> > physical addresses.
> > 
> > There's now the optional iommu_platform flag which can tell virtio
> > devices to use the platform's normal DMA path, including any IOMMUs.
> > That flag was motiviated for the case of hardware virtio
> > implementations, but there are other reasons to want it.
> > 
> > Specifically, the fact that the virtio device doesn't use vIOMMU
> > translation means that virtio devices are unsafe to pass to nested
> > guests, or to use with VFIO userspace drivers inside the guest.  This
> > is particularly noticeable on the pseries platform which *always* has
> > a guest-visible vIOMMU.
> > 
> > Not using the normal DMA path also causes difficulties for the guest
> > side driver when using the upcoming POWER Secure VMs (a.k.a. PEF).
> > While it's theoretically possible to handle this on the guest side,
> > it's really fiddly.  Given the other problems with the non-translated
> > virtio device, let's just enable vIOMMU translation for virtio devices
> > by default in the pseries-5.0 (and later) machine types.
> > 
> > This does mean the new machine type will no longer support guest
> > kernels older than 4.8, unless they have support for the virtio
> > IOMMU_PLATFORM flag backported (which some distro kernels like RHEL7
> > do).
> > 
> > Signed-off-by: David Gibson <address@hidden>
> > ---
> 
> The patch looks good but I'm not sure if we're quite ready to merge
> it yet. With this applied, I get zero output on a virtio-serial based
> console:
> 
> ie.
>   -chardev stdio,id=con0 -device virtio-serial -device 
> virtconsole,chardev=con0 
> 
> FYI, virtio-serial is a bit broken for spapr with iommu_platform=off already:
> 
> (1) pressing a key in the console during SLOF or grub has no effect
> 
> (2) the guest kernel boot stays stuck around quiesce
> 
> These are regressions introduced by this SLOF update:
> 
> a363e9ed8731f45674260932a340a0d81c4b0a6f is the first bad commit
> commit a363e9ed8731f45674260932a340a0d81c4b0a6f
> Author: Alexey Kardashevskiy <address@hidden>
> Date:   Tue Dec 17 11:31:54 2019 +1100
>     pseries: Update SLOF firmware image
> 
> A trivial fix was already posted on the SLOF list for (1) :
> 
> https://patchwork.ozlabs.org/patch/1249338/
> 
> (2) is still under investigation but the console is _at least_
> functional until the guest OS takes control. This is no longer
> the case with this patch.
> 

Some progress was made on the SLOF front:

https://patchwork.ozlabs.org/project/slof/list/?series=163314

With these series applied to SLOF, I can now boot a fedora31 guest
with a virtio-serial console and iommu_platform=on... but now
I'm trying out other virtio devices supported by SLOF and I'm
running into issues around virtio-pci.disable-legacy as mentioned
in some other mail...

It seems we may not be ready to merge this series yet.

> >  hw/ppc/spapr.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > index 3cfc98ac61..5ef099536e 100644
> > --- a/hw/ppc/spapr.c
> > +++ b/hw/ppc/spapr.c
> > @@ -4575,6 +4575,7 @@ static void 
> > spapr_machine_latest_class_options(MachineClass *mc)
> >       */
> >      static GlobalProperty compat[] = {
> >          { TYPE_VIRTIO_PCI, "disable-legacy", "on", },
> > +        { TYPE_VIRTIO_DEVICE, "iommu_platform", "on", },
> >      };
> >  
> >      mc->alias = "pseries";
> > @@ -4622,6 +4623,7 @@ static void 
> > spapr_machine_4_2_class_options(MachineClass *mc)
> >      SpaprMachineClass *smc = SPAPR_MACHINE_CLASS(mc);
> >      static GlobalProperty compat[] = {
> >          { TYPE_VIRTIO_PCI, "disable-legacy", "auto" },
> > +        { TYPE_VIRTIO_DEVICE, "iommu_platform", "off", },
> >      };
> >  
> >      spapr_machine_5_0_class_options(mc);
>