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Re: [Qemu-arm] [RFC] arm/cpu: fix soft lockup panic after resuming from

From: Heyi Guo
Subject: Re: [Qemu-arm] [RFC] arm/cpu: fix soft lockup panic after resuming from stop
Date: Thu, 11 Apr 2019 15:27:16 +0800
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.7.1

Hi Steve,

After reading kernel code about time keeping and something related, I've not 
got a clear picture of how we can use MSR_KVM_WALL_CLOCK_NEW to keep wall clock 
in guest VM.

1. On X86, MSR_KVM_WALL_CLOCK_NEW is only used by the callback of system 
suspend and resume; I didn't find it used for runtime wall clock reading.
2. To use the MSR for wall clock synchronization, shall we register KVM 
PV-clock as a higher rating clock source, so that it will be bound to 
tk_core.timekeeper and be read at each time of running update_wall_time() in 
each timer tick?
3. If the above is true, how can we keep the paravirtualized wall clock always 
updated? Is it always trapped to the hypervisor? I'm afraid this may cause 
performance loss. If there is no trap and the data is updated by the hypervisor 
periodically, how can we guarantee the accuracy?

Meanwhile it seems easier to use KVM_KVMCLOCK_CTRL to get rid of false positive soft lock panic, 
and guest can rely on cntvct for wall clock updating as it does now, and it seems not difficult for 
the hypervisor to keep cntvct "always on" and "monotonic".

Please let me know if I miss something.


On 2019/3/27 1:12, Steven Price wrote:
Hi Heyi,

On 26/03/2019 13:53, Heyi Guo wrote:
I also tested save/restore operations, and observed that clock in guest
would not jump after restoring either. If we consider guest clock not
being synchronized with real wall clock as an issue, does it mean
save/restore function has the same issue?
Basically at the moment when the guest isn't running you have a choice
of two behaviours:

  1. Stop (i.e. save/restore) CNTVCT - this means that the guest sees no
time occur. If the guest needs to have a concept of wall-clock time
(e.g. it communicates with other systems over a network) then this can
cause problems (e.g. timeouts might be wrong, certificates might start
appearing to be in the future etc).

  2. Leave CNTVCT running - the guest sees the time pass but interprets
the vCPUs as effectively having locked up. Linux will trigger the soft
lockup warning.

There are two ways of solving this, which match the two behaviours above:

  1. Provide the guest with a view of wall-clock time. The obvious way of
doing this is with a pvclock implementation like MSR_KVM_WALL_CLOCK_NEW
for x86.

  2. Inform the guest to ignore the apparent "soft-lockup". There's
already an ioctl for x86 for this: KVM_KVMCLOCK_CTRL

My preference is for option 1 - as this gives the guest a good view of
both the time that it is actually executing (useful for internal
watchdog timers like the soft-lockup one in Linux) and maintains a view
of wall-clock time (useful when communicating with other external
services - e.g. the a server on the internet). Your patch to QEMU
provides the first step of that, but as you mention there's much more to do.

One thing I haven't investigated in great detail is how KVM handles the
timer during various forms of suspend. In particular for suspend types
like full hibernation the host's physical counter will jump (quite
possibly backwards) - I haven't looked in detail how KVM presents this
to the guest. Hopefully not by making it go backwards!

I'm not sure how much time I'm going to have to look at this in the near
future, but please keep me in the loop if you decide to tackle any of this.




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