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Re: [Qemu-devel] [PATCH] kvmclock: clarify usage of cpu_clean_all_dirty
From: |
Marcelo Tosatti |
Subject: |
Re: [Qemu-devel] [PATCH] kvmclock: clarify usage of cpu_clean_all_dirty |
Date: |
Tue, 16 Sep 2014 13:48:24 -0300 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Tue, Sep 16, 2014 at 06:22:15PM +0200, Paolo Bonzini wrote:
> Il 16/09/2014 18:07, Marcelo Tosatti ha scritto:
> >> > The cpu_synchronize_all_states() call in kvmclock_vm_state_change() is
> >> > needed to make env->tsc up to date with the value on the source, right?
> > Its there to make sure the pair
> >
> > env->tsc, s->clock = data.clock
> >
> > are relative to point close in time.
>
> Ok. But why are they not close in time?
Scenario 1
A. s->clock = get_clock()
B. env->tsc = rdtsc()
Scenario 2
A. s->clock = get_clock()
C. VM callbacks, bdrv_flush, ...
B. env->tsc = rdtsc()
They are not "close in time" because of C.
> Could we have the opposite situation where env->tsc is loaded a long
> time _after_ s->clock, and something breaks?
This particular read avoids an overflow. See
+ assert(time.tsc_timestamp <= migration_tsc);
About your question, perhaps, would have to make up an
env->tsc, s->clock pair which breaks the code or a guest
application.
> Also, there is no reason to do kvmclock_current_nsec() during normal
> execution of the VM. Is the s->clock sent by the source ever good
> across migration, and could the source send kvmclock_current_nsec()
> instead of whatever KVM_GET_CLOCK returns?
Yes it could. What difference does it make?
> I don't understand this code very well, but it seems to me that the
> migration handling and VM state change handler are mixed up...
I don't see what the problem is. I am sure you can understand the code.
Re: [Qemu-devel] [PATCH] kvmclock: clarify usage of cpu_clean_all_dirty, Marcelo Tosatti, 2014/09/16