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Subject: Re: [Qemu-devel] [PATCH v2] kvmclock: Ensure time in migration
 never goes backward
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To: Alexander Graf <agraf@suse.de>, qemu-devel@nongnu.org
Cc: pbonzini@redhat.com

> +    cpu_physical_memory_read(kvmclock_struct_pa, &time, sizeof(time));
> +
> +    delta = migration_tsc - time.tsc_timestamp;

Hi,

when I was testing live storage migration with libvirt I found out that 
this patch can cause virtual machine to hang when completing mirror job.

This is (probably) because kvmclock_current_nsec() is called twice in a 
row and on second call time.tsc_timestamp is larger than migration_tsc. 
This causes delta to be huge and sets timer to invalid value.

The double call happens when switching from old to new disk (pivoting in 
libvirt's nomenclature).

Example values:

First call: migration_tsc: 12052203518652476, time_tsc: 
12052203301565676, delta 108543400

Second call: migration_tsc: 12052203518652476, time_tsc: 
12052204478600322, delta 9223372036374801885

Perhaps it is worth adding:

if (time.tsc_timestamp > migration_tsc) {
     return 0;
}

there? Untested though...

-- 
mg