Re: [Qemu-devel] KVM "fake DAX" flushing interface - discussion

[Pankaj Gupta]

Hello,

Thank you all for all the useful suggestions.
I want to summarize the discussions so far in the
thread. Please see below:

> >>
> >>> We can go with the "best" interface for what
> >>> could be a relatively slow flush (fsync on a
> >>> file on ssd/disk on the host), which requires
> >>> that the flushing task wait on completion
> >>> asynchronously.
> >>
> >>
> >> I'd like to clarify the interface of "wait on completion
> >> asynchronously" and KVM async page fault a bit more.
> >>
> >> Current design of async-page-fault only works on RAM rather
> >> than MMIO, i.e, if the page fault caused by accessing the
> >> device memory of a emulated device, it needs to go to
> >> userspace (QEMU) which emulates the operation in vCPU's
> >> thread.
> >>
> >> As i mentioned before the memory region used for vNVDIMM
> >> flush interface should be MMIO and consider its support
> >> on other hypervisors, so we do better push this async
> >> mechanism into the flush interface design itself rather
> >> than depends on kvm async-page-fault.
> > 
> > I would expect this interface to be virtio-ring based to queue flush
> > requests asynchronously to the host.
> 
> Could we reuse the virtio-blk device, only with a different device id?

As per previous discussions, there were suggestions on main two parts of the 
project:

1] Expose vNVDIMM memory range to KVM guest.

   - Add flag in ACPI NFIT table for this new memory type. Do we need NVDIMM 
spec 
     changes for this? 

   - Guest should be able to add this memory in system memory map. Name of the 
added memory in
     '/proc/iomem' should be different(shared memory?) than persistent memory 
as it 
     does not satisfy exact definition of persistent memory (requires an 
explicit flush).

   - Guest should not allow 'device-dax' and other fancy features which are not 
     virtualization friendly.

2] Flushing interface to persist guest changes.

   - As per suggestion by ChristophH (CCed), we explored options other then 
virtio like MMIO etc.
     Looks like most of these options are not use-case friendly. As we want to 
do fsync on a
     file on ssd/disk on the host and we cannot make guest vCPU's wait for that 
time. 

   - Though adding new driver(virtio-pmem) looks like repeated work and not 
needed so we can 
     go with the existing pmem driver and add flush specific to this new memory 
type.

   - Suggestion by Paolo & Stefan(previously) to use virtio-blk makes sense if 
just 
     want a flush vehicle to send guest commands to host and get reply after 
asynchronous
     execution. There was previous discussion [1] with Rik & Dan on this.

    [1] https://lists.gnu.org/archive/html/qemu-devel/2017-07/msg08373.html 

Is my understanding correct here?

Thanks,
Pankaj