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

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Fri, Jul 21, 2017 at 02:56:34AM -0400, Pankaj Gupta wrote:
> A] Problems to solve:
> ------------------
> 
> 1] We are considering two approaches for 'fake DAX flushing interface'.
>     
>  1.1] fake dax with NVDIMM flush hints & KVM async page fault
> 
>      - Existing interface.
> 
>      - The approach to use flush hint address is already nacked upstream.
>
>      - Flush hint not queued interface for flushing. Applications might 
>        avoid to use it.

This doesn't contradicts the last point about async operation and vcpu
control.  KVM async page faults turn the Address Flush Hints write into
an async operation so the guest can get other work done while waiting
for completion.

> 
>      - Flush hint address traps from guest to host and do an entire fsync 
>        on backing file which itself is costly.
> 
>      - Can be used to flush specific pages on host backing disk. We can 
>        send data(pages information) equal to cache-line size(limitation) 
>        and tell host to sync corresponding pages instead of entire disk sync.

Are you sure?  Your previous point says only the entire device can be
synced.  The NVDIMM Adress Flush Hints interface does not involve
address range information.

> 
>      - This will be an asynchronous operation and vCPU control is returned 
>        quickly.
> 
> 
>  1.2] Using additional para virt device in addition to pmem device(fake dax with device flush)

Perhaps this can be exposed via ACPI as part of the NVDIMM standards
instead of a separate KVM-only paravirt device.

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