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MIPS: KVM: Rewrite count/compare timer emulation

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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>
This commit is contained in:
James Hogan 2014-05-29 10:16:35 +01:00 committed by Paolo Bonzini
parent 3a0ba77408
commit e30492bbe9
4 changed files with 413 additions and 38 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

21
arch/mips/include/asm/kvm_host.h
View File

@ -404,8 +404,15 @@ struct kvm_vcpu_arch {
u32 io_gpr; /* GPR used as IO source/target */
/* Used to calibrate the virutal count register for the guest */
int32_t host_cp0_count;
struct hrtimer comparecount_timer;
/* Count bias from the raw time */
uint32_t count_bias;
/* Frequency of timer in Hz */
uint32_t count_hz;
/* Dynamic nanosecond bias (multiple of count_period) to avoid overflow */
s64 count_dyn_bias;
/* Period of timer tick in ns */
u64 count_period;
/* Bitmask of exceptions that are pending */
unsigned long pending_exceptions;
@ -426,8 +433,6 @@ struct kvm_vcpu_arch {
uint32_t guest_kernel_asid[NR_CPUS];
struct mm_struct guest_kernel_mm, guest_user_mm;
struct hrtimer comparecount_timer;
int last_sched_cpu;
/* WAIT executed */
@ -705,7 +710,13 @@ extern enum emulation_result kvm_mips_emulate_bp_exc(unsigned long cause,
extern enum emulation_result kvm_mips_complete_mmio_load(struct kvm_vcpu *vcpu,
struct kvm_run *run);
enum emulation_result kvm_mips_emulate_count(struct kvm_vcpu *vcpu);
uint32_t kvm_mips_read_count(struct kvm_vcpu *vcpu);
void kvm_mips_write_count(struct kvm_vcpu *vcpu, uint32_t count);
void kvm_mips_write_compare(struct kvm_vcpu *vcpu, uint32_t compare);
void kvm_mips_init_count(struct kvm_vcpu *vcpu);
void kvm_mips_count_enable_cause(struct kvm_vcpu *vcpu);
void kvm_mips_count_disable_cause(struct kvm_vcpu *vcpu);
enum hrtimer_restart kvm_mips_count_timeout(struct kvm_vcpu *vcpu);
enum emulation_result kvm_mips_check_privilege(unsigned long cause,
uint32_t *opc,

10
arch/mips/kvm/kvm_mips.c
View File

@ -363,7 +363,7 @@ struct kvm_vcpu *kvm_arch_vcpu_create(struct kvm *kvm, unsigned int id)
vcpu->arch.last_sched_cpu = -1;
/* Start off the timer */
kvm_mips_emulate_count(vcpu);
kvm_mips_init_count(vcpu);
return vcpu;
@ -707,9 +707,6 @@ static int kvm_mips_set_reg(struct kvm_vcpu *vcpu,
case KVM_REG_MIPS_CP0_STATUS:
kvm_write_c0_guest_status(cop0, v);
break;
case KVM_REG_MIPS_CP0_CAUSE:
kvm_write_c0_guest_cause(cop0, v);
break;
case KVM_REG_MIPS_CP0_EPC:
kvm_write_c0_guest_epc(cop0, v);
break;
@ -719,6 +716,7 @@ static int kvm_mips_set_reg(struct kvm_vcpu *vcpu,
/* registers to be handled specially */
case KVM_REG_MIPS_CP0_COUNT:
case KVM_REG_MIPS_CP0_COMPARE:
case KVM_REG_MIPS_CP0_CAUSE:
return kvm_mips_callbacks->set_one_reg(vcpu, reg, v);
default:
return -EINVAL;
@ -992,9 +990,7 @@ enum hrtimer_restart kvm_mips_comparecount_wakeup(struct hrtimer *timer)
vcpu = container_of(timer, struct kvm_vcpu, arch.comparecount_timer);
kvm_mips_comparecount_func((unsigned long) vcpu);
hrtimer_forward_now(&vcpu->arch.comparecount_timer,
ktime_set(0, MS_TO_NS(10)));
return HRTIMER_RESTART;
return kvm_mips_count_timeout(vcpu);
}
int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)

391
arch/mips/kvm/kvm_mips_emul.c
View File

@ -11,6 +11,7 @@
#include <linux/errno.h>
#include <linux/err.h>
#include <linux/ktime.h>
#include <linux/kvm_host.h>
#include <linux/module.h>
#include <linux/vmalloc.h>
@ -228,25 +229,364 @@ enum emulation_result update_pc(struct kvm_vcpu *vcpu, uint32_t cause)
return er;
}
/* Everytime the compare register is written to, we need to decide when to fire
* the timer that represents timer ticks to the GUEST.
/**
* kvm_mips_count_disabled() - Find whether the CP0_Count timer is disabled.
* @vcpu: Virtual CPU.
*
* Returns: 1 if the CP0_Count timer is disabled by the guest CP0_Cause.DC
* bit.
* 0 otherwise (in which case CP0_Count timer is running).
*/
enum emulation_result kvm_mips_emulate_count(struct kvm_vcpu *vcpu)
static inline int kvm_mips_count_disabled(struct kvm_vcpu *vcpu)
{
struct mips_coproc *cop0 = vcpu->arch.cop0;
enum emulation_result er = EMULATE_DONE;
return kvm_read_c0_guest_cause(cop0) & CAUSEF_DC;
}
/* If COUNT is enabled */
if (!(kvm_read_c0_guest_cause(cop0) & CAUSEF_DC)) {
hrtimer_try_to_cancel(&vcpu->arch.comparecount_timer);
hrtimer_start(&vcpu->arch.comparecount_timer,
ktime_set(0, MS_TO_NS(10)), HRTIMER_MODE_REL);
} else {
hrtimer_try_to_cancel(&vcpu->arch.comparecount_timer);
/**
* kvm_mips_ktime_to_count() - Scale ktime_t to a 32-bit count.
*
* Caches the dynamic nanosecond bias in vcpu->arch.count_dyn_bias.
*
* Assumes !kvm_mips_count_disabled(@vcpu) (guest CP0_Count timer is running).
*/
static uint32_t kvm_mips_ktime_to_count(struct kvm_vcpu *vcpu, ktime_t now)
{
s64 now_ns, periods;
u64 delta;
now_ns = ktime_to_ns(now);
delta = now_ns + vcpu->arch.count_dyn_bias;
if (delta >= vcpu->arch.count_period) {
/* If delta is out of safe range the bias needs adjusting */
periods = div64_s64(now_ns, vcpu->arch.count_period);
vcpu->arch.count_dyn_bias = -periods * vcpu->arch.count_period;
/* Recalculate delta with new bias */
delta = now_ns + vcpu->arch.count_dyn_bias;
}
return er;
/*
* We've ensured that:
* delta < count_period
*
* Therefore the intermediate delta*count_hz will never overflow since
* at the boundary condition:
* delta = count_period
* delta = NSEC_PER_SEC * 2^32 / count_hz
* delta * count_hz = NSEC_PER_SEC * 2^32
*/
return div_u64(delta * vcpu->arch.count_hz, NSEC_PER_SEC);
}
/**
* kvm_mips_read_count_running() - Read the current count value as if running.
* @vcpu: Virtual CPU.
* @now: Kernel time to read CP0_Count at.
*
* Returns the current guest CP0_Count register at time @now and handles if the
* timer interrupt is pending and hasn't been handled yet.
*
* Returns: The current value of the guest CP0_Count register.
*/
static uint32_t kvm_mips_read_count_running(struct kvm_vcpu *vcpu, ktime_t now)
{
ktime_t expires;
int running;
/* Is the hrtimer pending? */
expires = hrtimer_get_expires(&vcpu->arch.comparecount_timer);
if (ktime_compare(now, expires) >= 0) {
/*
* Cancel it while we handle it so there's no chance of
* interference with the timeout handler.
*/
running = hrtimer_cancel(&vcpu->arch.comparecount_timer);
/* Nothing should be waiting on the timeout */
kvm_mips_callbacks->queue_timer_int(vcpu);
/*
* Restart the timer if it was running based on the expiry time
* we read, so that we don't push it back 2 periods.
*/
if (running) {
expires = ktime_add_ns(expires,
vcpu->arch.count_period);
hrtimer_start(&vcpu->arch.comparecount_timer, expires,
HRTIMER_MODE_ABS);
}
}
/* Return the biased and scaled guest CP0_Count */
return vcpu->arch.count_bias + kvm_mips_ktime_to_count(vcpu, now);
}
/**
* kvm_mips_read_count() - Read the current count value.
* @vcpu: Virtual CPU.
*
* Read the current guest CP0_Count value, taking into account whether the timer
* is stopped.
*
* Returns: The current guest CP0_Count value.
*/
uint32_t kvm_mips_read_count(struct kvm_vcpu *vcpu)
{
struct mips_coproc *cop0 = vcpu->arch.cop0;
/* If count disabled just read static copy of count */
if (kvm_mips_count_disabled(vcpu))
return kvm_read_c0_guest_count(cop0);
return kvm_mips_read_count_running(vcpu, ktime_get());
}
/**
* kvm_mips_freeze_hrtimer() - Safely stop the hrtimer.
* @vcpu: Virtual CPU.
* @count: Output pointer for CP0_Count value at point of freeze.
*
* Freeze the hrtimer safely and return both the ktime and the CP0_Count value
* at the point it was frozen. It is guaranteed that any pending interrupts at
* the point it was frozen are handled, and none after that point.
*
* This is useful where the time/CP0_Count is needed in the calculation of the
* new parameters.
*
* Assumes !kvm_mips_count_disabled(@vcpu) (guest CP0_Count timer is running).
*
* Returns: The ktime at the point of freeze.
*/
static ktime_t kvm_mips_freeze_hrtimer(struct kvm_vcpu *vcpu,
uint32_t *count)
{
ktime_t now;
/* stop hrtimer before finding time */
hrtimer_cancel(&vcpu->arch.comparecount_timer);
now = ktime_get();
/* find count at this point and handle pending hrtimer */
*count = kvm_mips_read_count_running(vcpu, now);
return now;
}
/**
* kvm_mips_resume_hrtimer() - Resume hrtimer, updating expiry.
* @vcpu: Virtual CPU.
* @now: ktime at point of resume.
* @count: CP0_Count at point of resume.
*
* Resumes the timer and updates the timer expiry based on @now and @count.
* This can be used in conjunction with kvm_mips_freeze_timer() when timer
* parameters need to be changed.
*
* It is guaranteed that a timer interrupt immediately after resume will be
* handled, but not if CP_Compare is exactly at @count. That case is already
* handled by kvm_mips_freeze_timer().
*
* Assumes !kvm_mips_count_disabled(@vcpu) (guest CP0_Count timer is running).
*/
static void kvm_mips_resume_hrtimer(struct kvm_vcpu *vcpu,
ktime_t now, uint32_t count)
{
struct mips_coproc *cop0 = vcpu->arch.cop0;
uint32_t compare;
u64 delta;
ktime_t expire;
/* Calculate timeout (wrap 0 to 2^32) */
compare = kvm_read_c0_guest_compare(cop0);
delta = (u64)(uint32_t)(compare - count - 1) + 1;
delta = div_u64(delta * NSEC_PER_SEC, vcpu->arch.count_hz);
expire = ktime_add_ns(now, delta);
/* Update hrtimer to use new timeout */
hrtimer_cancel(&vcpu->arch.comparecount_timer);
hrtimer_start(&vcpu->arch.comparecount_timer, expire, HRTIMER_MODE_ABS);
}
/**
* kvm_mips_update_hrtimer() - Update next expiry time of hrtimer.
* @vcpu: Virtual CPU.
*
* Recalculates and updates the expiry time of the hrtimer. This can be used
* after timer parameters have been altered which do not depend on the time that
* the change occurs (in those cases kvm_mips_freeze_hrtimer() and
* kvm_mips_resume_hrtimer() are used directly).
*
* It is guaranteed that no timer interrupts will be lost in the process.
*
* Assumes !kvm_mips_count_disabled(@vcpu) (guest CP0_Count timer is running).
*/
static void kvm_mips_update_hrtimer(struct kvm_vcpu *vcpu)
{
ktime_t now;
uint32_t count;
/*
* freeze_hrtimer takes care of a timer interrupts <= count, and
* resume_hrtimer the hrtimer takes care of a timer interrupts > count.
*/
now = kvm_mips_freeze_hrtimer(vcpu, &count);
kvm_mips_resume_hrtimer(vcpu, now, count);
}
/**
* kvm_mips_write_count() - Modify the count and update timer.
* @vcpu: Virtual CPU.
* @count: Guest CP0_Count value to set.
*
* Sets the CP0_Count value and updates the timer accordingly.
*/
void kvm_mips_write_count(struct kvm_vcpu *vcpu, uint32_t count)
{
struct mips_coproc *cop0 = vcpu->arch.cop0;
ktime_t now;
/* Calculate bias */
now = ktime_get();
vcpu->arch.count_bias = count - kvm_mips_ktime_to_count(vcpu, now);
if (kvm_mips_count_disabled(vcpu))
/* The timer's disabled, adjust the static count */
kvm_write_c0_guest_count(cop0, count);
else
/* Update timeout */
kvm_mips_resume_hrtimer(vcpu, now, count);
}
/**
* kvm_mips_init_count() - Initialise timer.
* @vcpu: Virtual CPU.
*
* Initialise the timer to a sensible frequency, namely 100MHz, zero it, and set
* it going if it's enabled.
*/
void kvm_mips_init_count(struct kvm_vcpu *vcpu)
{
/* 100 MHz */
vcpu->arch.count_hz = 100*1000*1000;
vcpu->arch.count_period = div_u64((u64)NSEC_PER_SEC << 32,
vcpu->arch.count_hz);
vcpu->arch.count_dyn_bias = 0;
/* Starting at 0 */
kvm_mips_write_count(vcpu, 0);
}
/**
* kvm_mips_write_compare() - Modify compare and update timer.
* @vcpu: Virtual CPU.
* @compare: New CP0_Compare value.
*
* Update CP0_Compare to a new value and update the timeout.
*/
void kvm_mips_write_compare(struct kvm_vcpu *vcpu, uint32_t compare)
{
struct mips_coproc *cop0 = vcpu->arch.cop0;
/* if unchanged, must just be an ack */
if (kvm_read_c0_guest_compare(cop0) == compare)
return;
/* Update compare */
kvm_write_c0_guest_compare(cop0, compare);
/* Update timeout if count enabled */
if (!kvm_mips_count_disabled(vcpu))
kvm_mips_update_hrtimer(vcpu);
}
/**
* kvm_mips_count_disable() - Disable count.
* @vcpu: Virtual CPU.
*
* Disable the CP0_Count timer. A timer interrupt on or before the final stop
* time will be handled but not after.
*
* Assumes CP0_Count was previously enabled but now Guest.CP0_Cause.DC has been
* set (count disabled).
*
* Returns: The time that the timer was stopped.
*/
static ktime_t kvm_mips_count_disable(struct kvm_vcpu *vcpu)
{
struct mips_coproc *cop0 = vcpu->arch.cop0;
uint32_t count;
ktime_t now;
/* Stop hrtimer */
hrtimer_cancel(&vcpu->arch.comparecount_timer);
/* Set the static count from the dynamic count, handling pending TI */
now = ktime_get();
count = kvm_mips_read_count_running(vcpu, now);
kvm_write_c0_guest_count(cop0, count);
return now;
}
/**
* kvm_mips_count_disable_cause() - Disable count using CP0_Cause.DC.
* @vcpu: Virtual CPU.
*
* Disable the CP0_Count timer and set CP0_Cause.DC. A timer interrupt on or
* before the final stop time will be handled, but not after.
*
* Assumes CP0_Cause.DC is clear (count enabled).
*/
void kvm_mips_count_disable_cause(struct kvm_vcpu *vcpu)
{
struct mips_coproc *cop0 = vcpu->arch.cop0;
kvm_set_c0_guest_cause(cop0, CAUSEF_DC);
kvm_mips_count_disable(vcpu);
}
/**
* kvm_mips_count_enable_cause() - Enable count using CP0_Cause.DC.
* @vcpu: Virtual CPU.
*
* Enable the CP0_Count timer and clear CP0_Cause.DC. A timer interrupt after
* the start time will be handled, potentially before even returning, so the
* caller should be careful with ordering of CP0_Cause modifications so as not
* to lose it.
*
* Assumes CP0_Cause.DC is set (count disabled).
*/
void kvm_mips_count_enable_cause(struct kvm_vcpu *vcpu)
{
struct mips_coproc *cop0 = vcpu->arch.cop0;
uint32_t count;
kvm_clear_c0_guest_cause(cop0, CAUSEF_DC);
/*
* Set the dynamic count to match the static count.
* This starts the hrtimer.
*/
count = kvm_read_c0_guest_count(cop0);
kvm_mips_write_count(vcpu, count);
}
/**
* kvm_mips_count_timeout() - Push timer forward on timeout.
* @vcpu: Virtual CPU.
*
* Handle an hrtimer event by push the hrtimer forward a period.
*
* Returns: The hrtimer_restart value to return to the hrtimer subsystem.
*/
enum hrtimer_restart kvm_mips_count_timeout(struct kvm_vcpu *vcpu)
{
/* Add the Count period to the current expiry time */
hrtimer_add_expires_ns(&vcpu->arch.comparecount_timer,
vcpu->arch.count_period);
return HRTIMER_RESTART;
}
enum emulation_result kvm_mips_emul_eret(struct kvm_vcpu *vcpu)
@ -471,8 +811,7 @@ kvm_mips_emulate_CP0(uint32_t inst, uint32_t *opc, uint32_t cause,
#endif
/* Get reg */
if ((rd == MIPS_CP0_COUNT) && (sel == 0)) {
/* XXXKYMA: Run the Guest count register @ 1/4 the rate of the host */
vcpu->arch.gprs[rt] = (read_c0_count() >> 2);
vcpu->arch.gprs[rt] = kvm_mips_read_count(vcpu);
} else if ((rd == MIPS_CP0_ERRCTL) && (sel == 0)) {
vcpu->arch.gprs[rt] = 0x0;
#ifdef CONFIG_KVM_MIPS_DYN_TRANS
@ -539,10 +878,7 @@ kvm_mips_emulate_CP0(uint32_t inst, uint32_t *opc, uint32_t cause,
}
/* Are we writing to COUNT */
else if ((rd == MIPS_CP0_COUNT) && (sel == 0)) {
/* Linux doesn't seem to write into COUNT, we throw an error
* if we notice a write to COUNT
*/
/*er = EMULATE_FAIL; */
kvm_mips_write_count(vcpu, vcpu->arch.gprs[rt]);
goto done;
} else if ((rd == MIPS_CP0_COMPARE) && (sel == 0)) {
kvm_debug("[%#x] MTCz, COMPARE %#lx <- %#lx\n",
@ -552,7 +888,7 @@ kvm_mips_emulate_CP0(uint32_t inst, uint32_t *opc, uint32_t cause,
/* If we are writing to COMPARE */
/* Clear pending timer interrupt, if any */
kvm_mips_callbacks->dequeue_timer_int(vcpu);
kvm_write_c0_guest_compare(cop0,
kvm_mips_write_compare(vcpu,
vcpu->arch.gprs[rt]);
} else if ((rd == MIPS_CP0_STATUS) && (sel == 0)) {
kvm_write_c0_guest_status(cop0,
@ -564,6 +900,20 @@ kvm_mips_emulate_CP0(uint32_t inst, uint32_t *opc, uint32_t cause,
#ifdef CONFIG_KVM_MIPS_DYN_TRANS
kvm_mips_trans_mtc0(inst, opc, vcpu);
#endif
} else if ((rd == MIPS_CP0_CAUSE) && (sel == 0)) {
uint32_t old_cause, new_cause;
old_cause = kvm_read_c0_guest_cause(cop0);
new_cause = vcpu->arch.gprs[rt];
/* Update R/W bits */
kvm_change_c0_guest_cause(cop0, 0x08800300,
new_cause);
/* DC bit enabling/disabling timer? */
if ((old_cause ^ new_cause) & CAUSEF_DC) {
if (new_cause & CAUSEF_DC)
kvm_mips_count_disable_cause(vcpu);
else
kvm_mips_count_enable_cause(vcpu);
}
} else {
cop0->reg[rd][sel] = vcpu->arch.gprs[rt];
#ifdef CONFIG_KVM_MIPS_DYN_TRANS
@ -1553,8 +1903,7 @@ kvm_mips_handle_ri(unsigned long cause, uint32_t *opc,
current_cpu_data.icache.linesz);
break;
case 2: /* Read count register */
printk("RDHWR: Cont register\n");
arch->gprs[rt] = kvm_read_c0_guest_count(cop0);
arch->gprs[rt] = kvm_mips_read_count(vcpu);
break;
case 3: /* Count register resolution */
switch (current_cpu_data.cputype) {

27
arch/mips/kvm/kvm_trap_emul.c
View File

@ -407,8 +407,7 @@ static int kvm_trap_emul_get_one_reg(struct kvm_vcpu *vcpu,
{
switch (reg->id) {
case KVM_REG_MIPS_CP0_COUNT:
/* XXXKYMA: Run the Guest count register @ 1/4 the rate of the host */
*v = (read_c0_count() >> 2);
*v = kvm_mips_read_count(vcpu);
break;
default:
return -EINVAL;
@ -424,10 +423,30 @@ static int kvm_trap_emul_set_one_reg(struct kvm_vcpu *vcpu,
switch (reg->id) {
case KVM_REG_MIPS_CP0_COUNT:
/* Not supported yet */
kvm_mips_write_count(vcpu, v);
break;
case KVM_REG_MIPS_CP0_COMPARE:
kvm_write_c0_guest_compare(cop0, v);
kvm_mips_write_compare(vcpu, v);
break;
case KVM_REG_MIPS_CP0_CAUSE:
/*
* If the timer is stopped or started (DC bit) it must look
* atomic with changes to the interrupt pending bits (TI, IRQ5).
* A timer interrupt should not happen in between.
*/
if ((kvm_read_c0_guest_cause(cop0) ^ v) & CAUSEF_DC) {
if (v & CAUSEF_DC) {
/* disable timer first */
kvm_mips_count_disable_cause(vcpu);
kvm_change_c0_guest_cause(cop0, ~CAUSEF_DC, v);
} else {
/* enable timer last */
kvm_change_c0_guest_cause(cop0, ~CAUSEF_DC, v);
kvm_mips_count_enable_cause(vcpu);
}
} else {
kvm_write_c0_guest_cause(cop0, v);
}
break;
default:
return -EINVAL;