Re: [Qemu-devel] [PATCH v4 01/10] hw/mips/cputimer: Don't start periodic timer in KVM mode

[James Hogan]

On 19/03/14 16:29, Paolo Bonzini wrote:
> Il 14/03/2014 13:47, James Hogan ha scritto:
>> From: Sanjay Lal <address@hidden>
>>
>> Compare/Count timer interrupts are handled in-kernel for KVM, so don't
>> bother starting it in QEMU.
>>
>> Signed-off-by: Sanjay Lal <address@hidden>
>> Signed-off-by: James Hogan <address@hidden>
>> Reviewed-by: Aurelien Jarno <address@hidden>
>> ---
>> Changes in v2:
>>  - Expand commit message
>>  - Rebase on v1.7.0
>>  - Wrap comment
>> ---
>>  hw/mips/cputimer.c | 13 ++++++++++---
>>  1 file changed, 10 insertions(+), 3 deletions(-)
>>
>> diff --git a/hw/mips/cputimer.c b/hw/mips/cputimer.c
>> index c8b4b00..52570fd 100644
>> --- a/hw/mips/cputimer.c
>> +++ b/hw/mips/cputimer.c
>> @@ -23,6 +23,7 @@
>>  #include "hw/hw.h"
>>  #include "hw/mips/cpudevs.h"
>>  #include "qemu/timer.h"
>> +#include "sysemu/kvm.h"
>>
>>  #define TIMER_FREQ    100 * 1000 * 1000
>>
>> @@ -141,7 +142,13 @@ static void mips_timer_cb (void *opaque)
>>
>>  void cpu_mips_clock_init (CPUMIPSState *env)
>>  {
>> -    env->timer = timer_new_ns(QEMU_CLOCK_VIRTUAL, &mips_timer_cb, env);
>> -    env->CP0_Compare = 0;
>> -    cpu_mips_store_count(env, 1);
>> +    /*
>> +     * If we're in KVM mode, don't start the periodic timer, that is
>> handled in
>> +     * kernel.
>> +     */
>> +    if (!kvm_enabled()) {
>> +        env->timer = timer_new_ns(QEMU_CLOCK_VIRTUAL, &mips_timer_cb,
>> env);
>> +        env->CP0_Compare = 0;
>> +        cpu_mips_store_count(env, 1);
>> +    }
>>  }
>>
> 
> I hate to make you do unrelated changes, but... initializing CP0_Compare
> is unnecessary, it should already be 0;

You mean because of the memset in object_initialize_with_type, when
object_new is called? Although that wouldn't handle reset, although
technically the reset state of Compare is undefined.

> and for CP0_Count it should not
> be done here. but in cpu_state_reset function.  Then here you can call
> qemu_register_reset to register another reset callback, and call
> cpu_mips_timer_update in that callback.
> 
> I'm asking because while
> 
>     if (!kvm_enabled()) {
>         env->timer = ...
>         qemu_register_reset(...);
>     }
> 
> is fine, changing values of registers conditionally is not.

Okay, makes sense.

> 
> Also, I noticed two things in the implementation of the CPU timer that
> should be fixed:
> 
> 1) right now the hypervisor's frequency is hardcoded to 1/4th of the
> host, while QEMU's is 100 MHz.  It would be nice to make them either
> consistent, or customizable (you can use another ONE_REG interface to
> set CPU parameters).

Agreed. I'm in the middle of fixing the count/compare timer in KVM to be
based on real time (ktime_get()), so I'll make it default to 100MHz to
match QEMU for now. I can imagine it being useful to be able to control
it too depending on whether you're running on a slow FPGA/emulator or
fast silicon.

> 2) in KVM, CP0_Count does not start at the same value on guest reset.
> There is a comment that "Linux doesn't seem to write into COUNT", but
> QEMU does.  So KVM should implement CP0_Count writes and adjust the
> "bias" of the guest CP0_Count.

True, I hadn't considered qemu writing those registers yet.

Am I right that the correct way to prevent clock drift is for
kvm_arch_put_registers to only set the Count register if level !=
KVM_PUT_RUNTIME_STATE?

> In fact, right now kvm_mips_te_put_cp0_registers should always return
> -EINVAL because KVM_REG_MIPS_CP0_COUNT is not handled in
> kvm_mips_get/set_reg.  Am I missing something?

Yes, you appear to be right!

Thanks a lot for reviewing

Cheers
James

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