Re: [pve-devel] [PATCH pve-kernel] d/rules: kconfig: do not enable intel_iommu by default

[Stoiko Ivanov <s.ivanov@proxmox.com>]

On Fri, 13 May 2022 11:31:42 +0200
Thomas Lamprecht <t.lamprecht@proxmox.com> wrote:

> Am 5/13/22 um 10:57 schrieb Stoiko Ivanov:
> ..snip..
> > alternatively we could document this in the 7.2 release notes and ask
> > users of problematic hardware to set this explicitly  
> 
> This all resembles quite a bit the switch from nested KVM to be enabled,
> which we caught upfront and actively disabled to avoid trouble on migration
> due to QEMU not being yet ready for that change, and only enabled it in the
> next major release (6.0 IIRC) where we could be sure that a new enough QEMU
> is available.
> 
> The difference here is that:
> * we may never be sure that all setups out there support IOMMU, at least not
>   if we don't get another major divider like the switch from 32bit to 64bit
>   was
agreed - I think we want to switch to default enabled eventually - my idea
was to have it even more visible during a major version-upgrade ...
OTOH since we just released 7.2 and have a known-issues section - I think
adding it there and linking to that should work fine.

> 
> * we already switched it on now, sow disabling it may also break some setups
>   that never did so manually.
my gut-feeling would be that most people who need it enabled (for
pci-passtrough) would follow the docs (of the past years) and set it
explicitly [0] - but you're right - it's a regression we would need to
document too.

> FWIW, we could also disable this dynamically with a modprobe cfg file that
> disables it if:
> - not manually enabled already (grep for that)
> - an older CPU (family) is present, where the cutoff would need to be
>   decided

maybe I'm missing something - but intel_iommu is builtin (not a module) -
and options for those need to be provided on the kernel commandline [1]?

and while (semi-optionally) shipping a modprobe.conf snippet could work,
I wouldn't want to `sed` in our user's `/etc/default/grub`,
`/etc/kernel/cmdline` in a postinst.
we could add it to the NEWS file for the packages - to give it some
visibility though)?


[0] we should update our docs on pci-passthrough (mostly meant as note to
myself) 

[1]
https://www.kernel.org/doc/html/v4.14/admin-guide/kernel-parameters.html