Re: [PATCH] xen/time: fix gtime_to_gtsc for vtsc=1 PV guests

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 25/04/16 12:18, Stefano Stabellini wrote:
> From: Jan Beulich <JBeulich@suse.com>
>
> For vtsc=1 PV guests, rdtsc is trapped and calculated from get_s_time()
> using gtime_to_gtsc. Similarly the tsc_timestamp, part of struct
> vcpu_time_info, is calculated from stime_local_stamp using
> gtime_to_gtsc.
>
> However gtime_to_gtsc can return 0, if time < vtsc_offset, which can
> actually happen when gtime_to_gtsc is called passing stime_local_stamp
> (the caller function is __update_vcpu_system_time).
>
> In that case the pvclock protocol doesn't work properly and the guest is
> unable to calculate the system time correctly. As a consequence when the
> guest tries to set a timer event (for example calling the
> VCPUOP_set_singleshot_timer hypercall), the event will be in the past
> causing Linux to hang.
>
> The purpose of the pvclock protocol is to allow the guest to calculate
> the system_time in nanosec correctly. The guest calculates as follow:
>
>   from_vtsc_scale(rdtsc - vcpu_time_info.tsc_timestamp) + vcpu_time_info.system_time
>
> Given that with vtsc=1:
>   rdtsc = to_vtsc_scale(NOW() - vtsc_offset)
>   vcpu_time_info.tsc_timestamp = to_vtsc_scale(vcpu_time_info.system_time - vtsc_offset)
>
> The expression evaluates to NOW(), which is what we want.  However when
> stime_local_stamp < vtsc_offset, vcpu_time_info.tsc_timestamp is
> actually 0. As a consequence the calculated overall system_time is not
> correct.
>
> This patch fixes the issue by letting gtime_to_gtsc return a negative
> integer in the form of a wrapped around unsigned integer, thus when the
> guest subtracts vcpu_time_info.tsc_timestamp from rdtsc will calculate
> the right value.
>
> Signed-off-by: Jan Beulich <JBeulich@suse.com>
> Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>

Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

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