[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [XEN PATCH 1/7] xen: introduce XENFEAT_xenstore_late_init
On 11.01.2022 00:08, Stefano Stabellini wrote: > On Mon, 10 Jan 2022, Jan Beulich wrote: >> On 08.01.2022 01:49, Stefano Stabellini wrote: >>> Introduce a new feature flag to signal that xenstore will not be >>> immediately available at boot time. Instead, xenstore will become >>> available later, and a notification of xenstore readiness will be >>> signalled to the guest using the xenstore event channel. >> >> In addition to what Julien has said, I'd like to point out that Linux'es >> xenbus driver already has means to deal with xenstored not being around >> right away (perhaps because of living in a stubdom which starts in >> parallel). I therefore wonder whether what you want can't be achieved >> entirely inside that driver, without any new feature flag. > > The fewer changes to Linux the better but we couldn't find a way to make > it work with zero changes. > > In a way, the patch for Linux is re-using the existing infrastructure to > delay initialization, e.g. xenbus_probe_thread to call xenbus_probe > later. > > However, today there is nothing stopping Linux/HVM to continue > initialization immediately except for xs_hvm_defer_init_for_callback > which is not applicable in this case. So we introduced > XENFEAT_xenstore_late_init. > > As I wrote in another reply, I think we could do without > XENFEAT_xenstore_late_init if we instead rely on checking for > HVM_PARAM_STORE_EVTCHN being valid and HVM_PARAM_STORE_PFN being zero. Just as an aside - as discussed in some other context not so long ago, HVM_PARAM_*_PFN being zero isn't the best way of expressing "not yet initialized", and hence you would also want to check against ~0. > But either way as far as I can tell we need to add a new check to delay > xenstore initialization in Linux/HVM. Yes, I can see that a Linux side change might be needed. But isolating the change to there will be much better than needing to also have a Xen side change in place. Jan
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