[libvirt] [PATCH v3] Add support for fd: protocol

[Kevin Wolf]

Am 26.07.2011 16:00, schrieb Eric Blake:
> On 07/26/2011 06:51 AM, Corey Bryant wrote:
>> There are some additional features provided by certain image types
>> where Qemu reopens the image file. All of these scenarios will be
>> unsupported for the fd: protocol, at least for this patch:
>>
>>    - The -snapshot command line option
>>    - The savevm monitor command
>>    - The snapshot_blkdev monitor command
>>    - Use of copy-on-write image files
>>    - The -cdrom command line option
>>    - The -drive command line option with media=cdrom
>>    - The change monitor command
>>
>> The thought is that this support can be added in the future, but is
>> not required for the initial fd: support.
> 
> Libvirt will eventually need support for fd passing on savevm, 
> snapshot_blkdev, and change monitor commands, as well as for -cdrom, 
> before this feature can be used to provide the desired security 
> enhancements.  I agree that for an incremental patch, you don't have to 
> solve all points at once, but until all places have been modified to 
> support fd usage, you aren't gaining any security, except for severely 
> constrained guests.
> 
> Furthermore, how do you plan to map fd: to filename?  There's already 
> been big threads on why snapshot_blkdev needs both the new fd: and the 
> name of the old backing file at the same time, so that qemu can write 
> the correct headers into new qcow2 files. 

That's a problem to solve in snapshot_blkdev, not in -drive. In general
qemu doesn't need and shouldn't know the file name if it's meant to use
an fd.

Kevin