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

[Greg Kurz]

On Thu, 12 Mar 2020 15:14:06 +1100
Alexey Kardashevskiy <address@hidden> wrote:

> 
> 
> On 10/03/2020 21:43, Greg Kurz wrote:
> > 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.
> 
> 
> fwiw I sent a pull request:
> 
> https://lore.kernel.org/qemu-devel/address@hidden/T/#u
> 

Great ! Thanks mate ! :)

> 
> 
> > 
> >>>  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);
> >>
> > 
>