Sasha Levin <levinsasha...@gmail.com> writes: > On 07/22/2012 09:14 PM, Anthony Liguori wrote: >> Sasha Levin <levinsasha...@gmail.com> writes: >> >>> On 07/21/2012 10:44 AM, Wen Congyang wrote: >>>> We can know the guest is panicked when the guest runs on xen. >>>> But we do not have such feature on kvm. >>>> >>>> Another purpose of this feature is: management app(for example: >>>> libvirt) can do auto dump when the guest is panicked. If management >>>> app does not do auto dump, the guest's user can do dump by hand if >>>> he sees the guest is panicked. >>>> >>>> We have three solutions to implement this feature: >>>> 1. use vmcall >>>> 2. use I/O port >>>> 3. use virtio-serial. >>>> >>>> We have decided to avoid touching hypervisor. The reason why I choose >>>> choose the I/O port is: >>>> 1. it is easier to implememt >>>> 2. it does not depend any virtual device >>>> 3. it can work when starting the kernel >>> >>> Was the option of implementing a virtio-watchdog driver considered? >>> >>> You're basically re-implementing a watchdog, a guest-host interface and a >>> set of protocols for guest-host communications. >>> >>> Why can't we re-use everything we have now, push a virtio watchdog >>> driver into drivers/watchdog/, and gain a more complete solution to >>> detecting hangs inside the guest. >> >> The purpose of virtio is not to reinvent every possible type of device. >> There are plenty of hardware watchdogs that are very suitable to be used >> for this purpose. QEMU implements quite a few already. >> >> Watchdogs are not performance sensitive so there's no point in using >> virtio. > > The issue here is not performance, but the adding of a brand new > guest-host interface.
We have: 1) Virtio--this is our preferred PV interface. It needs PCI to be fully initialized and probably will live as a module. 2) Hypercalls--this a secondary PV interface but is available very early. It's terminated in kvm.ko which means it can only operate on things that are logically part of the CPU and/or APIC complex. This patch introduces a third interface which is available early like hypercalls but not necessarily terminated in kvm.ko. That means it can have a broader scope in functionality than (2). We could just as well use a hypercall and have multiple commands issued to that hypercall as a convention and add a new exit type to KVM that sent that specific hypercall to userspace for processing. But a PIO operation already has this behavior and requires no changes to kvm.ko. > virtio-rng isn't performance sensitive either, yet it was implemented > using virtio so there wouldn't be yet another interface to communicate > between guest and host. There isn't really an obvious discrete RNG that is widely supported. > This patch goes ahead to add a "arch pv features" interface using > ioports, without any idea what it might be used for beyond this > watchdog. It's not a watchdog--it's the opposite of a watchdog. You know such a thing already exists in the kernel, right? S390 has had a hypercall like this for years. Regards, Anthony Liguori
