Re: [kvm-devel] Guest kernel hangs in smp kvm for older kernels prior to tsc sync cleanup

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Avi Kivity wrote:
Ingo Molnar wrote:
While the change mentions that it fixes a time warp bug, it also says it should be rare. So clearly kvm smp tsc handing is buggy. Ingo/Thomas, (or anybody else), do you have any insight as to what kvm can be doing wrong to trigger this behavior?
hm. Those time warps were really small, due to the small imperfections in the "sync up all CPUs to the same moment and do a WRMSR to clear all their TSCs" mechanism. I.e. at most a few usec time warps. I really dont know how that should result in udevd hanging. Can you debug udevd in any way?


Adding debug didn't help. I'll try some sysrq keys to see what the guest thinks is happening.


many udev children are exiting; udevd itself is sleeping:

udevd         S D5DCDF24  2924   573    372   594     629   535 (NOTLB)
d5dcdf38 00000086 00000002 d5dcdf24 d5dcdf20 00000000 d5dcdefc d6169f68 d7db7f68 d5dcdf68 00000001 d5dd7560 c13b8a90 749ae8d2 00000002 000326a1 d5dd7684 c131c700 00000003 d74f8900 892d6946 00000402 ffffffff 00000000
Call Trace:
 [<c060d2c9>] do_nanosleep+0x3b/0x66
 [<c0439b20>] hrtimer_nanosleep+0x50/0x106
 [<c04397ee>] hrtimer_wakeup+0x0/0x18
 [<c0439c1f>] sys_nanosleep+0x49/0x59
 [<c0404e4c>] syscall_call+0x7/0xb
 [<c0600000>] xfrm_state_find+0x49f/0x51e

So likely sleeping is screwed up somehow (though only on smp).


so the only thing that KVM might be doing incorrectly here is the emulation of the WRMSR that clears the TSC of each vcpu?

By inspection, it is correct. Of course I may be missing something, so I'll write a unit test for it. It should also be much slower than the native wrmsr.


Testing shows wrmsr and rdtsc function normally.

I'll try pinning the vcpus to cpus and see if that helps.

--
error compiling committee.c: too many arguments to function

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