LoongArch: KVM: Do not restart SW timer when it is expired

LoongArch VCPUs have their own separate HW timers. SW timer is to wake
up blocked vcpu thread, rather than HW timer emulation. When blocking
vcpu scheduled out, SW timer is used to wakeup blocked vcpu thread and
injects timer interrupt. It does not care about whether guest timer is
in period mode or oneshot mode, and SW timer needs not to be restarted
since vcpu has been woken.

This patch does not restart SW timer when it is expired.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: KVM: Start SW timer only when vcpu is blocking

SW timer is enabled when vcpu thread is scheduled out, and it is to wake
up vcpu from blocked queue. If vcpu thread is scheduled out but is not
blocked, such as it is preempted by other threads, it is not necessary
to enable SW timer. Since vcpu thread is still on running queue if it is
preempted and SW timer is only to wake up vcpu on blocking queue, so SW
timer is not useful in this situation.

This patch enables SW timer only when vcpu is scheduled out and is
blocking.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: KVM: Fix timer emulation with oneshot mode

When timer is fired in oneshot mode, CSR TVAL will be -1 rather than 0.
There needs special handing for this situation. There are two scenarios
when oneshot timer is fired. One scenario is that time is fired after
exiting to host, CSR TVAL is set with 0 in order to inject hw interrupt,
and -1 will assigned to CSR TVAL soon.

The other situation is that timer is fired in VM and guest kernel is
hanlding timer IRQ, IRQ is acked and is ready to set next expired timer
value, then vm exits to host. Timer interrupt should not be inject at
this point, else there will be spurious timer interrupt.

Here hw timer irq status in CSR ESTAT is used to judge these two
scenarios. If CSR TVAL is -1, the oneshot timer is fired; and if timer
hw irq is on in CSR ESTAT register, it happens after exiting to host;
else if timer hw irq is off, we think that it happens in vm and timer
IRQ handler has already acked IRQ.

With this patch, runltp with version ltp20230516 passes to run in vm.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: KVM: Remove kvm_acquire_timer() before entering guest

Timer emulation method in VM is switch to SW timer, there are two
places where timer emulation is needed. One is during vcpu thread
context switch, the other is halt-polling with idle instruction
emulation. SW timer switching is removed during halt-polling mode,
so it is not necessary to disable SW timer before entering to guest.

This patch removes SW timer handling before entering guest mode, and
put it in HW timer restoring flow when vcpu thread is sched-in. With
this patch, vm timer emulation is simpler, there is SW/HW timer
switch only in vcpu thread context switch scenario.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: KVM: Allow to access HW timer CSR registers always

Currently HW timer CSR registers are allowed to access before entering
to vm and disabled if switch to SW timer in host mode, instead it is not
necessary to do so. HW timer CSR registers can be accessed always, it
is nothing to do with whether it is in vm mode or host mode. This patch
removes the limitation.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: KVM: Remove SW timer switch when vcpu is halt polling

With halt-polling supported, there is checking for pending events or
interrupts when vcpu executes idle instruction. Pending interrupts
include injected SW interrupts and passthrough HW interrupts, such as
HW timer interrupts, since HW timer works still even if vcpu exists from
VM mode.

Since HW timer pending interrupt can be set directly with CSR status
register, and pending HW timer interrupt checking is used in vcpu block
checking function, it is not necessary to switch to SW timer during
halt-polling. This patch adds preemption disabling in function
kvm_cpu_has_pending_timer(), and removes SW timer switching in idle
instruction emulation function.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

LoongArch: KVM: Implement vcpu timer operations

Implement LoongArch vcpu timer operations such as init kvm timer,
acquire kvm timer, save kvm timer and restore kvm timer. When vcpu
exit, we use kvm soft timer to emulate hardware timer. If timeout
happens, the vcpu timer interrupt will be set and it is going to be
handled at vcpu next entrance.

Reviewed-by: Bibo Mao <maobibo@loongson.cn>
Tested-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Tianrui Zhao <zhaotianrui@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>