Re: [PATCH 2/4] x86: always initialize xen-swiotlb when xen-pcifront is enabling

[Andrew Cooper]

On 19/05/2023 1:41 pm, Christoph Hellwig wrote:
> On Fri, May 19, 2023 at 12:10:26PM +0200, Marek Marczykowski-Górecki wrote:
>> While I would say PCI passthrough is not very common for PV guests, can
>> the decision about xen-swiotlb be delayed until you can enumerate
>> xenstore to check if there are any PCI devices connected (and not
>> allocate xen-swiotlb by default if there are none)? This would
>> still not cover the hotplug case (in which case, you'd need to force it
>> with a cmdline), but at least you wouldn't loose much memory just
>> because one of your VMs may use PCI passthrough (so, you have it enabled
>> in your kernel).
> How early can we query xenstore?  We'd need to do this before setting
> up DMA for any device.

Not that early.  One supported configuration has xenstore not starting
for an indefinite period of time after boot.

> The alternative would be to finally merge swiotlb-xen into swiotlb, in
> which case we might be able to do this later.  Let me see what I can
> do there.

If that is an option, it would be great to reduce the special-cashing.

~Andrew