On Thu, Feb 24 2022, Alex Williamson <alex.williamson@xxxxxxxxxx> wrote: > On Thu, 24 Feb 2022 12:13:30 -0400 > Jason Gunthorpe <jgg@xxxxxxxxxx> wrote: > >> On Thu, Feb 24, 2022 at 08:30:42AM -0700, Alex Williamson wrote: >> > On Thu, 24 Feb 2022 16:21:11 +0100 >> > Cornelia Huck <cohuck@xxxxxxxxxx> wrote: >> > > conflicts with it. We should not create the impression that STOP_COPY >> > > will neccessarily be mandatory for all time. >> >> We really *should* create that impression because a userspace that >> does not test STOP_COPY in the cases required above is *broken* and >> must be strongly discouraged from existing. Well yes, you need STOP_COPY with the current implementation. I'm not arguing against that. >> >> The purpose of this comment is to inform the userspace implementator, >> not to muse about possible future expansion options for kernel >> developers. We all agree this expansion path exists and is valid, we >> need to keep that option open by helping userspace implement >> correctly. > > Chatting with Connie offline, I think the clarification that might help > is something alone the lines that the combination of bits must support > migration, which currently requires the STOP_COPY and RESUMING states. > The VFIO_MIGRATION_P2P flag alone does not provide these states. The > only flag in the current specification to provide these states is > VFIO_MIGRATION_STOP_COPY. I don't think we want to preclude that some > future flag might provide variants of STOP_COPY and RESUMING, so it's > not so much that VFIO_MIGRATION_STOP_COPY is mandatory, but it is > currently the only flag which provides the base degree of migration > support. Indeed. > > How or if that translates to an actual documentation update, I'm not > sure. As it stands, we're not speculating about future support, we're > only stating these two combinations are valid. Future combinations may > or may not include VFIO_MIGRATION_STOP_COPY. As the existing proposed > comment indicates, other combinations are TBD. Connie? Thanks, > > Alex Hm... "a flag indicating support for a migration state machine such as VFIO_MIGRATION_STOP_COPY is mandatory"?
