Re: [RFC PATCH v2 0/4] vfio/pci: Atomic Ops completer support

[Robin Voetter <robin@streamhpc.com>]

On 6/1/23 00:24, Alex Williamson wrote:
> On Wed, 31 May 2023 23:55:41 +0200
> Robin Voetter <robin@streamhpc.com> wrote:
>> Something that I have been thinking about, are there any implications
>> involved with enabling this feature automatically with no possibility of
>> turning it off? I have no use case for that, though, and I cant really
>> think of a reason other than preventing the guest from finding out
>> hardware details about the host.
> 
> Not sure I follow, are you suggesting that Atomic Ops completion
> support is proactively enabled in the VM to match the host, regardless
> of attached assigned devices?  An obvious problem there is migration.
> If I start a VM on a host with Atomic Ops support and want to migrate
> to a host without Atomic Ops support, config space of the root ports is
> now different and the migration fails.  QEMU would also require
> elevated privileges to read config space to determine the host support,
> and then what does it do if only some of the PCIe hierarchy supports
> Atomic Ops.
> 
> Policy decisions like that are generally left to management tools, so
> there would always be a means to enable or disable the feature.  In
> fact, that's specifically why I test that the Atomic Op completer bits
> are unset in the root port before changing them so that this automatic
> enablement could live alongside a command line option to statically
> enable some bits.
> 
> That does however remind me that it is often good with these sorts of
> "clever" automatic features to have an opt-out, so I'll likely add an
> x-no-rp-atomics device option in the next version to provide that.

Yes, something like that that is exactly what I was thinking about.

Thanks,

Robin Voetter