Re: [Xen-devel] [PATCH v2 2/2] x86/HVM: Use fixed TSC value when saving or restoring domain

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 04/01/2014 11:53 AM, Boris Ostrovsky 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.
>
> (As part of the patch, update rdtscll's definition so that its variables don't
> clash with outer code)
>
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>
This patch will need to be updated as it doesn't quite fix the problem 
on some AMD processors (they may do one extra rdtsc in 
svm_set_tsc_offset()).
-boris


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