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
