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

[Tian, Kevin]

> From: Alex Williamson [mailto:address@hidden
> Sent: Wednesday, January 27, 2016 6:27 AM
> 
> 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,
> 

Could you elaborate this thought? If it can achieve the purpose w/o
a kernel exit definitely we can adapt to it. :-)

Thanks
Kevin