On Fri, Nov 1, 2024 at 1:23 PM Qiang Zhang <qiang4.zhang@xxxxxxxxxxxxxxx> wrote: > > On Fri, Nov 01, 2024 at 10:11:11AM +0800, Jason Wang wrote: > > On Fri, Nov 1, 2024 at 9:54 AM <qiang4.zhang@xxxxxxxxxxxxxxx> wrote: > > > > > > From: Qiang Zhang <qiang4.zhang@xxxxxxxxx> > > > > > > Virtio core unconditionally reset and restore status for all virtio > > > devices before calling restore method. This breaks some virtio drivers > > > which don't need to do anything in suspend and resume because they > > > just want to keep device state retained. > > > > The challenge is how can driver know device doesn't need rest. > > Hi, > > Per my understanding to PM, in the suspend flow, device drivers need to > 1. First manage/stop accesses from upper level software and > 2. Store the volatile context into in-memory data structures. > 3. Put devices into some low power (suspended) state. > The resume process does the reverse. > If a device context won't loose after entering some low power state > (optional), it's OK to skip step 2. > > For virtio devices, spec doesn't define whether their states will lost > after platform entering suspended state. This is exactly what suspend patch tries to define. > So to work with different > hypervisors, virtio drivers typically trigger a reset in suspend/resume > flow. This works fine for virtio devices if following conditions are met: > - Device state can be totally recoverable. > - There isn't any working behaviour expected in suspended state, i.e. the > suspended state should be sub-state of reset. > However, the first point may be hard to implement from driver side for some > devices. The second point may be unacceptable for some kind of devices. > > For your question, for devices whose suspended state is alike reset state, > the hypervisor have the flexibility to retain its state or not, kernel > driver can unconditionally reset it with proper re-initialization to > accomplish better compatibility. For others, hypervisor *must* retain > device state and driver just keeps using it. Right, so my question is how did the driver know the behaviour of a device? We usually do that via a feature bit. Note that the thing that matters here is the migration compatibility. > > > > > For example, PCI has no_soft_reset which has been done in the commit > > "virtio: Add support for no-reset virtio PCI PM". > > > > And there's a ongoing long discussion of adding suspend support in the > > virtio spec, then driver know it's safe to suspend/resume without > > reset. > > That's great! Hopefully it can fill the gap. > Currently, I think we can safely move the reset to drivers' freeze methods, > virtio core has no reason to take it as a common action required by all > devices. And the reset operation can be optional skipped if driver have > hints from device that it can retain state. The problem here is whether the device can be resumed without "soft reset" seems a general feature which could be either the knowledge of 1) virtio core (a feature bit or not) or 2) transport layer (like PCI) > > > > > > > > > Virtio GPIO is a typical example. GPIO states should be kept unchanged > > > after suspend and resume (e.g. output pins keep driving the output) and > > > Virtio GPIO driver does nothing in freeze and restore methods. But the > > > reset operation in virtio_device_restore breaks this. > > > > Is this mandated by GPIO or virtio spec? If yes, let's quote the revelant part. > > No. But in actual hardware design (e.g. Intel PCH GPIO), or from the > requirement perspective, GPIO pin state can be (should support) retained > in suspended state. > If Virtio GPIO is used to let VM operate such physical GPIO chip indirectly, > it can't be reset in suspend and resume. Meanwhile the hypervisor will > retain pin states after suspension. > > > > > > > > > Since some devices need reset in suspend and resume while some needn't, > > > create a new helper function for the original reset and status restore > > > logic so that virtio drivers can invoke it in their restore method > > > if necessary. > > > > How are those drivers classified? > > I think this depends whether hypervisor will keep devices state in platform > suspend process. So the problem is that the actual implementation (hypervisor, physical device or mediation) is transparent to the driver. Driver needs a general way to know whether it's safe (or not) to reset during the suspend/resume. > I think hypervisor should because suspend and reset are > conceptually two different things. Probably, but rest is and doing software state load/save is common practice for devices that will lose their state during PM. Thanks > > > Thanks > Qiang >
