On Wed, Jul 23, 2025 at 12:51:42AM -0300, Jason Gunthorpe wrote: > On Tue, Jul 22, 2025 at 10:49:10AM +1000, Alistair Popple wrote: > > > So what is it? > > > > IMHO a hack, because obviously we shouldn't require real physical addresses > > for > > something the CPU can't actually address anyway and this causes real > > problems > > IMHO what DEVICE PRIVATE really boils down to is a way to have swap > entries that point to some kind of opaque driver managed memory. > > We have alot of assumptions all over about pfn/phys to page > relationships so anything that has a struct page also has to come with > a fake PFN today..
Hmm ... maybe. To get that PFN though we have to come from either a special swap entry which we already have special cases for, or a struct page (which is a device private page) which we mostly have to handle specially anyway. I'm not sure there's too many places that can sensibly handle a fake PFN without somehow already knowing it is device-private PFN. > > (eg. it doesn't actually work on anything other than x86_64). There's no > > reason > > the "PFN" we store in device-private entries couldn't instead just be an > > index > > into some data structure holding pointers to the struct pages. So instead of > > using pfn_to_page()/page_to_pfn() we would use > > device_private_index_to_page() > > and page_to_device_private_index(). > > It could work, but any of the pfn conversions would have to be tracked > down.. Could be troublesome. I looked at this a while back and I'm reasonably optimistic that this is doable because we already have to treat these specially everywhere anyway. The proof will be writing the patches of course. - Alistair > Jason
