[virt-tools-list] [virt-manager 0/8] filesystem: Add support for virtiofs

[Cole Robinson]

On 7/15/21 9:17 AM, Michal Prívozník wrote:
> On 7/7/21 3:31 PM, Cole Robinson wrote:
>> On 7/7/21 4:50 AM, Michal Prívozník wrote:
>>> On 7/5/21 5:20 AM, lma wrote:
>>>> 在 2021-07-01 08:39，Michal Prívozník 写道：
>>>>> On 7/1/21 12:52 AM, Cole Robinson wrote:
>>>>>> (ccing mprivozn with a domaincapabilities design question below)
> 
> <snip/>
> 
>>
>> IMO just source type= is enough.
>>
> 
> FYI, I just merged patches that implement this in libvirt:
> 
> 133d05a15e qemu: capabilities: fill in domcaps <memoryBacking>
> e27e22585a conf: domcaps: Report <memoryBacking>
> 
> # virsh domcapabilities | xmllint --xpath
> /domainCapabilities/memoryBacking -
>   <memoryBacking supported="yes">
>     <enum name="sourceType">
>       <value>file</value>
>       <value>anonymous</value>
>       <value>memfd</value>
>     </enum>
>   </memoryBacking>
> 
> 

Thanks Michal!

We had some internal discussion about using memfd by default, but
there's some caveats: memfd implies shared memory, and shared memory
does things like prevent KSM from working, add some hiccups to memory
ballooning, and causes VM to appear to own all its memory after
save/restore. So I'm not sure we should set this as the default any time
soon.

So this puts us back at the beginning. Do we make addhardware try to
transparently set up shared memory when virtiofs is requested? That
still makes me leery TBH.

I'm thinking we should add a UI checkbox in the memory screen for
enabling/disabling shared memory. This way the logic is testable for an
existing VM outside of the addhardware wizard. If our logic is incorrect
and screws up their VM like making it unbootable, there's an explicit
way to back out that change. We should prefer memfd over file shm memory
if memfd is available in domcaps.

Then when adding virtiofs device in addhardware, we try to make the
default behavior work, but let the user opt out. So in the standard
case, where VM has no shared memory config, when selecting virtio-fs, we
show a new checkbox 'Enable shared memory for the whole VM' with an info
icon + tooltip explaining this is necessary for virtio-fs. We still show
it even when the host already has shared memory configured, but disable
the checkbox and add a tooltip saying 'VM is already configured with
shared memory'. I think that should be enough to make this safe to
maintain, and more informative to the user about what is happening

Thanks,
Cole