Re: [PATCH v3 2/6] KVM: Add KVM_CREATE_COALESCED_MMIO_BUFFER ioctl

[Sean Christopherson <seanjc@google.com>]

On Thu, Aug 29, 2024, Ilias Stamatis wrote:
> On Wed, 2024-08-28 at 07:25 -0700, Sean Christopherson wrote:
> > > returns a file descriptor to the caller but does not allocate a ring
> > > buffer. Userspace can then pass this fd to mmap() to actually allocate a
> > > buffer and map it to its address space.
> > > 
> > > Subsequent patches will allow userspace to:
> > > 
> > > - Associate the fd with a coalescing zone when registering it so that
> > >   writes to that zone are accumulated in that specific ring buffer
> > >   rather than the VM-wide one.
> > > - Poll for MMIO writes using this fd.
> > 
> > Why?  I get the desire for a doorbell, but KVM already supports "fast" I/O for
> > doorbells. 
> 
> What do you refer to here? ioeventfd? 

Ya.

> Correct me if I am wrong, but my understanding is that with an
> ioeventfd the write value is not available. And that is a problem when
> that value is a head or tail pointer. 

Ah.  Can you describe (or point at) an example device?  I don't read many device
specs (understatement).  It would be helpful to have a concrete use case for
reviewing the design itself.

In a perfect world, poll() support would come from a third party file type, as
this doesn't seem _that_ unique (to KVM).  But AFAICT there isn't an existing
type that is a good fit, probably because it's such a simple thing.