Re: [libvirt PATCH 2/3] qemu: Implement VIR_MIGRATE_ASSUME_SHARED_STORAGE support

On Tue, Oct 31, 2023 at 06:12:59PM +0100, Andrea Bolognani wrote: > if ((rc = virFileIsSharedFS(src)) < 0) { > return false; > } else if (rc == 0) { > + /* Ignore the outcome of this check if we've been > + * asked to assume that storage is shared */ > + if (flags & VIR_MIGRATE_ASSUME_SHARED_STORAGE) > + break; So this is essentially telling this particular bit of code to ignore what virFileIsSharedFS reported. The problem this is not the only piece of code relevant to live migration that cares about the virFileIsSharedFS answer. The TPM code also checks virFileIsSharedFS in the context of migration. Also in the security manager, we check virFileIsSharedFS and skip restoring ownership on the src host, as that would kill access on the dst host. So while VIR_MIGRATE_ASSUME_SHARED_STORAGE may work in the precise scenario you've tested, it looks like an incomplete solution to the described problem, and liable to cause new problems.

I feel like the root problem here needs addressing in the virFileIsSharedFS method in some manner. Whether or not a given location is shared between two hosts cannot be reliably determined from stat() alone, we need some other inputs. Maybe this is as simple as having a qemu.conf parameter shared_storage_paths = [ "/some/dir", "/other/dir"....] and then we register those paths via virFile during startup. Extending virFileIsSharedFS to work correctly in special deployment scenarios can now be done by the admin when configuring the host.