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MIPS: KVM: Rewrite count/compare timer emulation
Previously the emulation of the CPU timer was just enough to get a Linux
guest running but some shortcuts were taken:
- The guest timer interrupt was hard coded to always happen every 10 ms
rather than being timed to when CP0_Count would match CP0_Compare.
- The guest's CP0_Count register was based on the host's CP0_Count
register. This isn't very portable and fails on cores without a
CP_Count register implemented such as Ingenic XBurst. It also meant
that the guest's CP0_Cause.DC bit to disable the CP0_Count register
took no effect.
- The guest's CP0_Count register was emulated by just dividing the
host's CP0_Count register by 4. This resulted in continuity problems
when used as a clock source, since when the host CP0_Count overflows
from 0x7fffffff to 0x80000000, the guest CP0_Count transitions
discontinuously from 0x1fffffff to 0xe0000000.
Therefore rewrite & fix emulation of the guest timer based on the
monotonic kernel time (i.e. ktime_get()). Internally a 32-bit count_bias
value is added to the frequency scaled nanosecond monotonic time to get
the guest's CP0_Count. The frequency of the timer is initialised to
100MHz and cannot yet be changed, but a later patch will allow the
frequency to be configured via the KVM_{GET,SET}_ONE_REG ioctl
interface.
The timer can now be stopped via the CP0_Cause.DC bit (by the guest or
via the KVM_SET_ONE_REG ioctl interface), at which point the current
CP0_Count is stored and can be read directly. When it is restarted the
bias is recalculated such that the CP0_Count value is continuous.
Due to the nature of hrtimer interrupts any read of the guest's
CP0_Count register while it is running triggers a check for whether the
hrtimer has expired, so that the guest/userland cannot observe the
CP0_Count passing CP0_Compare without queuing a timer interrupt. This is
also taken advantage of when stopping the timer to ensure that a pending
timer interrupt is queued.
This replaces the implementation of:
- Guest read of CP0_Count
- Guest write of CP0_Count
- Guest write of CP0_Compare
- Guest write of CP0_Cause
- Guest read of HWR 2 (CC) with RDHWR
- Host read of CP0_Count via KVM_GET_ONE_REG ioctl interface
- Host write of CP0_Count via KVM_SET_ONE_REG ioctl interface
- Host write of CP0_Compare via KVM_SET_ONE_REG ioctl interface
- Host write of CP0_Cause via KVM_SET_ONE_REG ioctl interface
Signed-off-by: 
James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Gleb Natapov <gleb@kernel.org>
Cc: kvm@vger.kernel.org
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: Sanjay Lal <sanjayl@kymasys.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>