target/riscv: Stop timer with infinite timecmp

target/riscv: Stop timer with infinite timecmp

While the spec doesn't state it, setting timecmp to UINT64_MAX is
another way to stop a timer, as it's considered setting the next
timer event to occur at infinity. And, even if the time CSR does
eventually reach UINT64_MAX, the very next tick will bring it back to
zero, once again less than timecmp. For this reason
riscv_timer_write_timecmp() special cases UINT64_MAX. However, if a
previously set timecmp has not yet expired, then setting timecmp to
UINT64_MAX to disable / stop it would not work, as the special case
left the previous QEMU timer active, which would then still deliver
an interrupt at that previous timecmp time. Ensure the stopped timer
will not still deliver an interrupt by also deleting the QEMU timer
in the UINT64_MAX special case.

Fixes: ae0edf2188b3 ("target/riscv: No need to re-start QEMU timer when timecmp == UINT64_MAX")
Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240829084002.1805006-2-ajones@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>