Date: Thu, 15 Dec 2016 13:38:11 -0800 From: John Baldwin <jhb@freebsd.org> To: Roger Pau =?ISO-8859-1?Q?Monn=E9?= <roger.pau@citrix.com> Cc: freebsd-arch@freebsd.org Subject: Re: Order of device suspend/resume Message-ID: <7469755.xT5lfhErkd@ralph.baldwin.cx> In-Reply-To: <20161215114033.r33nt3fqhnfi7hqw@dhcp-3-221.uk.xensource.com> References: <20161215114033.r33nt3fqhnfi7hqw@dhcp-3-221.uk.xensource.com>
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On Thursday, December 15, 2016 11:40:33 AM Roger Pau Monné wrote: > Hello, > > I'm currently dealing with a bug in the Xen suspend/resume sequence, and I've > found that lacking a way to order device priority during suspend/resume is > proving quite harmful for Xen (and maybe other systems too). The current > suspend/resume code simply scans the root bus, and suspends/resumes every device > based on the order they are attached to their parents. The problem here is that > there's no way to tell that some devices should be resumed before others, for > example the event timers/time counters/uarts should definitely be resume before > other devices, but that's seems to happens mostly out of chance. > > Currently most time related devices are attached directly to the nexus, which > means they will get resumed first, but for example the uart is currently > attached to the pci bus IIRC, which means it gets resumed quite late. On Xen > systems, this is even worse. The Xen PV bus (that contains all Xen-related > devices) is attached the last one (because it tends to pick up unused memory > regions for it's own usage) and this bus also contains the PV timecounter which > should be resumed _before_ other devices, or else timecounting will be > completely screwed and things can get stuck in indefinitely long loops (due to > the fact that the timecounter is implemented based on the uptime of the host, > and that changes from host-to-host). > > In order to solve this I could add a hack to the Xen resume process (which is > already different from the ACPI one), but this looks gross. I could also attach > the Xen PV timer to the nexus directly (as it was done before), but I also > prefer to keep all Xen-related devices in the same bus for coherency. Last > option would be to add some kind of suspend/resume priorities to the devices, > and do more than one suspend/resume pass. This is more complex and requires more > changes, so I would like to know if it would be helpful for other systems, or if > someone has already attempted to do it. I think Justin Hibbits had some patches to make use of the boot-time new-bus passes for suspend and resume which I think would help with this. You suspend things in the reverse order of boot and resume operates in the same order as boot. -- John Baldwin
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