Re: [Xen-devel] [PATCH v4 3/3] x86/HVM: Use fixed TSC value when saving or restoring domain

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.04.14 at 03:27, <boris.ostrovsky@xxxxxxxxxx> wrote:
> When a domain is saved each VCPU's TSC value needs to be preserved. To get it 
> we
> use hvm_get_guest_tsc(). This routine (either itself or via get_s_time() 
> which
> it may call) calculates VCPU's TSC based on current host's TSC value (by 
> doing a
> rdtscll()). Since this is performed for each VCPU separately we end up with
> un-synchronized TSCs.
> 
> Similarly, during a restore each VCPU is assigned its TSC based on host's 
> current
> tick, causing virtual TSCs to diverge further.
> 
> With this, we can easily get into situation where a guest may see time going
> backwards.
> 
> Instead of reading new TSC value for each VCPU when saving/restoring it we 
> should
> use the same value across all VCPUs.
> 
> Reported-by: Philippe Coquard <philippe.coquard@xxxxxxxx>
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>



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