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Re: [PATCH v2 0/2] spapr: Use vIOMMU translation for virtio by default

From: Greg Kurz
Subject: Re: [PATCH v2 0/2] spapr: Use vIOMMU translation for virtio by default
Date: Thu, 13 Feb 2020 12:46:43 +0100

On Thu, 13 Feb 2020 11:58:35 +1100
David Gibson <address@hidden> wrote:

> Upcoming Secure VM support for pSeries machines introduces some
> complications for virtio, since the transfer buffers need to be
> explicitly shared so that the hypervisor can access them.
> While it's not strictly speaking dependent on it, the fact that virtio
> devices bypass normal platform IOMMU translation complicates the issue
> on the guest side.  Since there are some significan downsides to
> bypassing the vIOMMU anyway, let's just disable that.
> There's already a flag to do this in virtio, just turn it on by
> default for forthcoming pseries machine types.
> Any opinions on whether dropping support for the older guest kernels
> is acceptable at this point?

As expected, this breaks compatibility with existing RHEL 6.10 guests. Each
patch in this series requires an extra -global option to be specified on
the command line in order to boot successfully.

Patch 1: -global virtio-pci.disable-legacy=auto
Patch 2: -global virtio-pci.iommu_platform=off

As seen on the RH site [1], RHEL6 will reach "End of Maintenance Support
or Maintenance Support 2 (Product retirement)" on November 30, 2020 and
"End of Extended Life-cycle Support" on June 30, 2024.

Not sure if it's okay to drop support for RHEL6 this soon.

RHEL 7.7 guests seem to be unaffected.

[1] https://access.redhat.com/support/policy/updates/errata/#Life_Cycle_Dates

> Changes since v1:
>  * Added information on which guest kernel versions will no longer
>    work with these changes
>  * Use Michael Tsirkin's suggested better way of handling the machine
>    type change
> David Gibson (2):
>   spapr: Disable legacy virtio devices for pseries-5.0 and later
>   spapr: Enable virtio iommu_platform=on by default
>  hw/ppc/spapr.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)

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