Re: [Qemu-devel] VFIO based vGPU(was Re: [Announcement] 2015-Q3 release of XenGT - a Mediated ...)

[Alex Williamson]

On Tue, 2016-01-26 at 22:15 +0000, Tian, Kevin wrote:
> > From: Alex Williamson [mailto:address@hidden
> > 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