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

[Greg Kurz <groug@kaod.org>]

On Thu,  5 Mar 2020 15:30:09 +1100
David Gibson <david@gibson.dropbear.id.au> 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 <david@gibson.dropbear.id.au>
> ---

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 <aik@ozlabs.ru>
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.

>  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);