Re: [Qemu-block] [PATCH v2 10/10] file-posix: Make auto-read-only dynamic

[Eric Blake]

On 3/11/19 11:50 AM, Kevin Wolf wrote:
> Until now, with auto-read-only=on we tried to open the file read-write
> first and if that failed, read-only was tried. This is actually not good
> enough for libvirt, which gives QEMU SELinux permissions for read-write
> only as soon as it actually intends to write to the image. So we need to
> be able to switch between read-only and read-write at runtime.
> 
> This patch makes auto-read-only dynamic, i.e. the file is opened
> read-only as long as no user of the node has requested write
> permissions, but it is automatically reopened read-write as soon as the
> first writer is attached. Conversely, if the last writer goes away, the
> file is reopened read-only again.
> 
> bs->read_only is no longer set for auto-read-only=on files even if the
> file descriptor is opened read-only because it will be transparently
> upgraded as soon as a writer is attached. This changes the output of
> qemu-iotests 232.

auto-read-only was introduced in 3.1, at which point we intentionally
had sufficiently loose wording to permit (but not require) dynamic state
checking; so you are not breaking the interface.  On the other hand, is
libvirt going to have problems introspecting whether it can use
auto-read-only and get the dynamic behavior it needs?  Or is there
enough else in the way of libvirt's switch to -blockdev that it won't
attempt anything that needs auto-read-only without other 4.0 interfaces
anyway, at which point detecting the presence of the field (but not
whether the field has a guarantee of dynamic behavior) on 3.1 doesn't
matter?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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