On 5/2/23 9:04 AM, Christian Borntraeger wrote: > > > Am 02.05.23 um 14:54 schrieb Lorenzo Stoakes: >> On Tue, May 02, 2023 at 02:46:28PM +0200, Christian Borntraeger wrote: >>> Am 02.05.23 um 01:11 schrieb Lorenzo Stoakes: >>>> Writing to file-backed dirty-tracked mappings via GUP is inherently broken >>>> as we cannot rule out folios being cleaned and then a GUP user writing to >>>> them again and possibly marking them dirty unexpectedly. >>>> >>>> This is especially egregious for long-term mappings (as indicated by the >>>> use of the FOLL_LONGTERM flag), so we disallow this case in GUP-fast as >>>> we have already done in the slow path. >>> >>> Hmm, does this interfer with KVM on s390 and PCI interpretion of interrupt delivery? >>> It would no longer work with file backed memory, correct? >>> >>> See >>> arch/s390/kvm/pci.c >>> >>> kvm_s390_pci_aif_enable >>> which does have >>> FOLL_WRITE | FOLL_LONGTERM >>> to >>> >> >> Does this memory map a dirty-tracked file? It's kind of hard to dig into where >> the address originates from without going through a ton of code. In worst case >> if the fast code doesn't find a whitelist it'll fall back to slow path which >> explicitly checks for dirty-tracked filesystem. > > It does pin from whatever QEMU uses as backing for the guest. >> >> We can reintroduce a flag to permit exceptions if this is really broken, are you >> able to test? I don't have an s390 sat around :) > > Matt (Rosato on cc) probably can. In the end, it would mean having > <memoryBacking> > <source type="file"/> > </memoryBacking> > > In libvirt I guess. I am running with this series applied using a QEMU guest with memory-backend-file (using the above libvirt snippet) for a few different PCI device types and AEN forwarding (e.g. what is setup in kvm_s390_pci_aif_enable) is still working.
