Modern hardware all supports a TSC_DEADLINE mode for the APIC timer,
where the same GHz-scale 64 bit TSC used for performance monitoring
becomes the free-running counter used for cpu-local timer interrupts.
Being a free running counter that does not need to be reset, it will
not lose time in an interrupt. Being 64 bit, it needs no rollover or
clamping logic in the driver when presented with a 32 bit tick count.
Being a proper comparator, it will correctly trigger interrupts for
times set "in the past" and thus needs no minimum/clamping logic. The
counter is synchronized across the system architecturally (modulo one
burp where firmware likes to change the adjustment value) so usage is
SMP-safe by default. Access to the 64 bit counter and comparator
value are single-instruction atomics even on 32 bit systems, so it
beats even the RISC-V machine timer in complexity (which was our
reigning champ for "simplest timer driver").
Really this is just ideal for Zephyr. So rather than try to add
support for it to the existing APIC driver and increase complexity,
make this a new standalone driver instead. All modern hardware has
what it needs. The sole gotcha is that it's not easily emulatable
(qemu supports it only under kvm where they can freeload on the host
TSC) so it can be exercised only on hardware platforms right now.
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
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