[PATCH v2 2/9] qemu: Introduced VIR_MIGRATE_TPM_SHARED_STORAGE for TPM migration

[Michal Prívozník]

On 10/17/22 17:17, Stefan Berger wrote:
> 
> 
> On 10/17/22 09:48, Daniel P. Berrangé wrote:
>> On Mon, Oct 17, 2022 at 09:39:52AM -0400, Stefan Berger wrote:
>>>
>>>
> 
>>
>> The key is in qemuMigrationSrcIsSafe(), and how it determines if a
>> migration is safe.
>>
>>    * Disk on local storage, no flags  => unsafe, migration error
>>    * Disk on local storage, VIR_MIGRATE_NON_SHARED_DISK => ok, copies
>> disk storage
>>    * Disk on shared storage, no flags => safe
>>    * Disk on shared storage, VIR_MIGRATE_NON_SHARED_DISK => ok, but
>> needlessly copies disk storage
>>
>> The key helper methods are virFileIsSharedFS and virFileIsClusterFS
>> which check the filesystem type for the path against a known list
>> of shared/cluster FS.
>>
>>> So we won't do it this way for TPM state migration. Instead we can
>>> try to write on the source migration side
>>>
>>> a) a simple file and detect whether the file is at the destination
>>> b) a file with either a name or content that only the source and
>>>     destination libvirts would know at this point
>>>
>>> b) is to prevent stale files from becoming indicators for shared
>>> storage.
>>
>> No, please use the virFileIsSharedFS/ClusterFS helpers.
>>
> 
> I tried to use virFileIsSharedFS on the source and destination side of
> my NFS setup to see how they work. The issue here is that the NFS server
> side, that exports /var/lib/libvirt/swtpm and is also the migration
> source at some point, says this:
> 
> /var/lib/libvirt/swtpm/ecc221c4-6bb9-423f-ac31-72244fdbb1a1/tpm2 is
> shared: 0
> 
> the NFS client side then says this:
> 
> 
> /var/lib/libvirt/swtpm/ecc221c4-6bb9-423f-ac31-72244fdbb1a1/tpm2 is
> shared: 1
> 

Are both sides mounted as NFS or just one of them? I think it's safe to
assume that either both sides are on a shared FS or none. Otherwise I'm
quite sure file locking won't work anyways. We can go even further and
test whether shared_fs(source) == shared_fs(dst); and throw an error if
it doesn't.

And since I've misguided you on the way with flag I am willing to help
you write patches. Please let me know if you need any help.

Michal