On Tue, 2016-01-26 at 22:15 +0000, Tian, Kevin wrote: > > From: Alex Williamson [mailto:alex.william...@redhat.com] > > Sent: Wednesday, January 27, 2016 6:08 AM > > > > > > > > > > > > > > > > > > Today KVMGT (not using VFIO yet) registers I/O emulation callbacks to > > > > > KVM, so VM MMIO access will be forwarded to KVMGT directly for > > > > > emulation in kernel. If we reuse above R/W flags, the whole emulation > > > > > path would be unnecessarily long with obvious performance impact. We > > > > > either need a new flag here to indicate in-kernel emulation (bias from > > > > > passthrough support), or just hide the region alternatively (let KVMGT > > > > > to handle I/O emulation itself like today). > > > > > > > > That sounds like a future optimization TBH. There's very strict > > > > layering between vfio and kvm. Physical device assignment could make > > > > use of it as well, avoiding a round trip through userspace when an > > > > ioread/write would do. Userspace also needs to orchestrate those kinds > > > > of accelerators, there might be cases where userspace wants to see those > > > > transactions for debugging or manipulating the device. We can't simply > > > > take shortcuts to provide such direct access. Thanks, > > > > > > > > > > But we have to balance such debugging flexibility and acceptable > > > performance. > > > To me the latter one is more important otherwise there'd be no real usage > > > around this technique, while for debugging there are other alternative > > > (e.g. > > > ftrace) Consider some extreme case with 100k traps/second and then see > > > how much impact a 2-3x longer emulation path can bring... > > > > Are you jumping to the conclusion that it cannot be done with proper > > layering in place? Performance is important, but it's not an excuse to > > abandon designing interfaces between independent components. Thanks, > > > > Two are not controversial. My point is to remove unnecessary long trip > as possible. After another thought, yes we can reuse existing read/write > flags: > - KVMGT will expose a private control variable whether in-kernel > delivery is required;
But in-kernel delivery is never *required*. Wouldn't userspace want to deliver in-kernel any time it possibly could? > - when the variable is true, KVMGT will register in-kernel MMIO > emulation callbacks then VM MMIO request will be delivered to KVMGT > directly; > - when the variable is false, KVMGT will not register anything. > VM MMIO request will then be delivered to Qemu and then ioread/write > will be used to finally reach KVMGT emulation logic; No, that means the interface is entirely dependent on a backdoor through KVM. Why can't userspace (QEMU) do something like register an MMIO region with KVM handled via a provided file descriptor and offset, couldn't KVM then call the file ops without a kernel exit? Thanks, Alex
