Re: [PATCH RFC dontapply 5/5] kvm_para: guest side for eoi avoidance

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Apr 24, 2012 at 09:50:02AM +0300, Gleb Natapov wrote:
> On Mon, Apr 23, 2012 at 05:04:40PM +0300, Michael S. Tsirkin wrote:
> > The idea is simple: there's a bit, per APIC, in guest memory,
> > that tells the guest that it does not need EOI.
> > Guest tests it using a single est and clear operation - this is
> > necessary so that host can detect interrupt nesting - and if set, it can
> > skip the EOI MSR.
> > 
> > This patch is not final yet, pls don't apply
> > until host side is also finalized. Including
> > it here for completeness to show another user
> > of the new eoi_write interface.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Just pointing out the above: this last patch in the
series is for illustrative purposes at this point.

> > ---
> >  arch/x86/include/asm/bitops.h   |    6 +++-
> >  arch/x86/include/asm/kvm_para.h |    2 +
> >  arch/x86/kernel/kvm.c           |   51 ++++++++++++++++++++++++++++++++++++--
> >  3 files changed, 54 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
> > index b97596e..c9c70ea 100644
> > --- a/arch/x86/include/asm/bitops.h
> > +++ b/arch/x86/include/asm/bitops.h
> > @@ -26,11 +26,13 @@
> >  #if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 1)
> >  /* Technically wrong, but this avoids compilation errors on some gcc
> >     versions. */
> > -#define BITOP_ADDR(x) "=m" (*(volatile long *) (x))
> > +#define BITOP_ADDR_CONSTRAINT "=m"
> >  #else
> > -#define BITOP_ADDR(x) "+m" (*(volatile long *) (x))
> > +#define BITOP_ADDR_CONSTRAINT "+m"
> >  #endif
> >  
> > +#define BITOP_ADDR(x) BITOP_ADDR_CONSTRAINT (*(volatile long *) (x))
> > +
> >  #define ADDR				BITOP_ADDR(addr)
> >  
> >  /*
> > diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
> > index 734c376..195840d 100644
> > --- a/arch/x86/include/asm/kvm_para.h
> > +++ b/arch/x86/include/asm/kvm_para.h
> > @@ -22,6 +22,7 @@
> >  #define KVM_FEATURE_CLOCKSOURCE2        3
> >  #define KVM_FEATURE_ASYNC_PF		4
> >  #define KVM_FEATURE_STEAL_TIME		5
> > +#define KVM_FEATURE_EOI			6
> >  
> >  /* The last 8 bits are used to indicate how to interpret the flags field
> >   * in pvclock structure. If no bits are set, all flags are ignored.
> > @@ -37,6 +38,7 @@
> >  #define MSR_KVM_SYSTEM_TIME_NEW 0x4b564d01
> >  #define MSR_KVM_ASYNC_PF_EN 0x4b564d02
> >  #define MSR_KVM_STEAL_TIME  0x4b564d03
> > +#define MSR_KVM_EOI_EN      0x4b564d04
> >  
> >  struct kvm_steal_time {
> >  	__u64 steal;
> > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> > index b8ba6e4..ad66cdd 100644
> > --- a/arch/x86/kernel/kvm.c
> > +++ b/arch/x86/kernel/kvm.c
> > @@ -39,6 +39,8 @@
> >  #include <asm/desc.h>
> >  #include <asm/tlbflush.h>
> >  #include <asm/idle.h>
> > +#include <asm/apic.h>
> > +#include <asm/apicdef.h>
> >  
> >  static int kvmapf = 1;
> >  
> > @@ -290,6 +292,27 @@ static void kvm_register_steal_time(void)
> >  		cpu, __pa(st));
> >  }
> >  
> > +static DEFINE_PER_CPU(u16, kvm_apic_eoi) __aligned(2) = 0;
> > +
> > +/* Our own copy of __test_and_clear_bit to make sure
> > + * it is done with a single instruction */
> > +static inline int kvm_test_and_clear_bit(int nr, volatile u16* addr)
> > +{
> > +	int oldbit;
> > +
> > +	asm volatile("btr %2,%1\n\t"
> > +		     "sbb %0,%0"
> > +		     : "=r" (oldbit), BITOP_ADDR_CONSTRAINT(*addr) : "Ir" (nr));
> > +	return oldbit;
> > +}
> > +
> > +static void kvm_guest_apic_eoi_write(u32 reg, u32 val)
> > +{
> > +	if (kvm_test_and_clear_bit(0, &__get_cpu_var(kvm_apic_eoi)))
> > +		return;
> > +	apic->write(APIC_EOI, APIC_EOI_ACK);
> > +}
> > +
> >  void __cpuinit kvm_guest_cpu_init(void)
> >  {
> >  	if (!kvm_para_available())
> > @@ -307,11 +330,17 @@ void __cpuinit kvm_guest_cpu_init(void)
> >  		       smp_processor_id());
> >  	}
> >  
> > +	if (kvm_para_has_feature(KVM_FEATURE_EOI)) {
> > +		__get_cpu_var(kvm_apic_eoi) = 0;
> > +		wrmsrl(MSR_KVM_EOI_EN, __pa(&__get_cpu_var(kvm_apic_eoi)) |
> > +		       KVM_MSR_ENABLED);
> > +	}
> > +
> >  	if (has_steal_clock)
> >  		kvm_register_steal_time();
> >  }
> >  
> > -static void kvm_pv_disable_apf(void *unused)
> > +static void kvm_pv_disable_apf(void)
> >  {
> >  	if (!__get_cpu_var(apf_reason).enabled)
> >  		return;
> > @@ -323,11 +352,18 @@ static void kvm_pv_disable_apf(void *unused)
> >  	       smp_processor_id());
> >  }
> >  
> > +static void kvm_pv_guest_cpu_reboot(void *unused)
> > +{
> > +	if (kvm_para_has_feature(KVM_FEATURE_EOI))
> > +		wrmsrl(MSR_KVM_EOI_EN, 0);
> > +	kvm_pv_disable_apf();
> > +}
> > +
> >  static int kvm_pv_reboot_notify(struct notifier_block *nb,
> >  				unsigned long code, void *unused)
> >  {
> >  	if (code == SYS_RESTART)
> > -		on_each_cpu(kvm_pv_disable_apf, NULL, 1);
> > +		on_each_cpu(kvm_pv_guest_cpu_reboot, NULL, 1);
> >  	return NOTIFY_DONE;
> >  }
> >  
> > @@ -378,7 +414,9 @@ static void __cpuinit kvm_guest_cpu_online(void *dummy)
> >  static void kvm_guest_cpu_offline(void *dummy)
> >  {
> >  	kvm_disable_steal_time();
> > -	kvm_pv_disable_apf(NULL);
> > +	if (kvm_para_has_feature(KVM_FEATURE_EOI))
> > +		wrmsrl(MSR_KVM_EOI_EN, 0);
> > +	kvm_pv_disable_apf();
> >  	apf_task_wake_all();
> >  }
> >  
> > @@ -431,6 +469,13 @@ void __init kvm_guest_init(void)
> >  		pv_time_ops.steal_clock = kvm_steal_clock;
> >  	}
> >  
> > +	if (kvm_para_has_feature(KVM_FEATURE_EOI)) {
> > +		/* Should happen once after apic is initialized */
> > +		BUG_ON(!apic);
> What if kernel was started with noapic?

I didn't test this yet. I thought noapic sets the apic pointer to
apic_noop, not to NULL.

So this case will get slowed down slightly if it
actually runs on kvm. It does not look like noapic
guest is worth optimising for.

> > +		BUG_ON(apic->eoi_write == kvm_guest_apic_eoi_write);
> > +		apic->eoi_write = kvm_guest_apic_eoi_write;
> > +	}
> > +
> >  #ifdef CONFIG_SMP
> >  	smp_ops.smp_prepare_boot_cpu = kvm_smp_prepare_boot_cpu;
> >  	register_cpu_notifier(&kvm_cpu_notifier);
> > -- 
> > 1.7.9.111.gf3fb0
> 
> --
> 			Gleb.