Re: [PATCH v3 00/26] virtiofs dax patches

[Stefan Hajnoczi]

On Wed, Apr 28, 2021 at 12:00:34PM +0100, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> 
>   This series adds support for acceleration of virtiofs via DAX
> mapping, using features added in the 5.11 Linux kernel.
> 
>   DAX originally existed in the kernel for mapping real storage
> devices directly into memory, so that reads/writes turn into
> reads/writes directly mapped into the storage device.
> 
>   virtiofs's DAX support is similar; a PCI BAR is exposed on the
> virtiofs device corresponding to a DAX 'cache' of a user defined size.
> The guest daemon then requests files to be mapped into that cache;
> when that happens the virtiofsd sends filedescriptors and commands back
> to the QEMU that mmap's those files directly into the memory slot
> exposed to kvm.  The guest can then directly read/write to the files
> exposed by virtiofs by reading/writing into the BAR.
> 
>   A typical invocation would be:
>      -device 
> vhost-user-fs-pci,queue-size=1024,chardev=char0,tag=myfs,cache-size=4G
> 
> and then the guest must mount with -o dax
> 
>   Note that the cache doesn't really take VM up on the host, because
> everything placed there is just an mmap of a file, so you can afford
> to use quite a large cache size.
> 
>   Unlike a real DAX device, the cache is a finite size that's
> potentially smaller than the underlying filesystem (especially when
> mapping granuality is taken into account).  Mapping, unmapping and
> remapping must take place to juggle files into the cache if it's too
> small.  Some workloads benefit more than others.
> 
> Gotchas:
>   a) If something else on the host truncates an mmap'd file,
> kvm gets rather upset;  for this reason it's advised that DAX is
> currently only suitable for use on non-shared filesystems.
> 
> (This series, with a couple of other patches, is at:
> https://gitlab.com/virtio-fs/qemu/-/tree/dgilbert-dax-2021-04-28 )

Overall this looks close but I don't think the FSETID support should be
added to QEMU. Please see my comment on the final patch.

Stefan

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