[Xen-devel] arm/vtimer: Physical timer emulation and the physical counter

[Jeff Kubascik <jeff.kubascik@xxxxxxxxxxxxxxx>]

Hello,

I'm working on a port of a RTOS (RTEMS) to Xen on ARM, and came across an
interesting finding in how Xen emulates the physical timer on ARM.

In testing different configurations of the port, I have the RTOS configured to
use the ARM generic physical timer. The driver operates the physical timer in
the "CompareView" mode, where the timer condition is met when the physical
counter reaches the programmed CompareValue.

The driver initializes the physical timer by first reading the physical counter
register CNTPCT, adding the systick interval, and then writing the result to the
CompareValue register CNTP_CVAL. This appears to be valid behavior based on my
understanding of the ARMV8 Architecture Reference Manual, since the physical
timer "offset" is specified to be zero.

Xen will trap accesses to the physical timer registers - CNTP_CTL, CNTP_CVAL,
and CNTP_TVAL, which happens in xen/arch/arm/vtimer.c. Xen will add or remove an
offset phys_timer_base.offset when reading or writing to the CNTP_CVAL/CNTP_TVAL
registers. This offset is determined when the vtimer is initialized on guest
creation.

However, Xen does not trap access to the physical counter register CNTPCT. This
means the guest has direct access to the register. It also means the offset is
not applied here. I believe this is a problem, because the physical timer is no
longer consistent with the physical counter from the guest's perspective - there
is a non-zero, unknown offset between the two.

This was a problem for the RTOS, since it was reading the physical counter
register (Xen does not apply an offset), adding some interval, and then setting
the CompareValue register (Xen applies the offset), resulting in a long delay
before the timer expires.

I was able to fix this by adding code in Xen to trap access to CNTPCT and
applying the offset - I can submit the patch if there is interest. However, I
was curious if there was an reason for not trapping/ emulating access to the
physical counter register and applying the offset?

Sincerely,
Jeff Kubascik

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