Re: [Qemu-devel] Para-virtualized ram-based filesystem?

[Avi Kivity <avi@redhat.com>]

On 04/18/2011 06:28 AM, Ritchie, Stuart wrote:
> On 4/17/11 5:43 AM, "Avi Kivity"<avi@redhat.com>  wrote:
>
> >On 04/16/2011 02:58 AM, Ritchie, Stuart wrote:
> >>  >
> >>  >You can do this with ivshmem today.  You give it a path to a shared
> >>  >memory file, and then there's a path in sysfs that you can mmap() in
> >>  >userspace in the guest.
> >>
> >>  Please correct me if I am wrong, but with ivshmem you must to manage
> >>your
> >>  world within a single, fixed size region.  I appreciate the simplicity
> >>of
> >>  mapping the whole region all in one go, but our requirements are a bit
> >>  different.  Even if you could pass multiple -device ivshmem instances,
> >>  it's still a fixed environment.  Right?
> >>
> >
> >You could place a read-only filesystem (say iso9660) in the region and
> >mount it; it will then appear as a complete filesystem.
>
> We've thought about formatting the region as a ramdisk, but the block
> layer shields mmap() from the storage, thus requiring a data copy into the
> page-cache.  The great thing about ramfs/tmpfs is the data is used
> in-place; we'd lose that when going with a ramdisk or other real
> filesystem.

s390 uses a trick to achieve this (XIP).

Look at fs/ext2/xip.c.

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