Re: [PATCHv7 3/8] kvm_para: guest side for eoi avoidance

Michael S. Tsirkin
2012-06-18T14:51:01Z

On Mon, Jun 18, 2012 at 05:17:24PM +0300, Avi Kivity wrote:
> On 06/14/2012 04:53 PM, 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.
> >
> > I run a simple microbenchmark to show exit reduction
> > (note: for testing, need to apply follow-up patch
> > 'kvm: host side for eoi optimization' + a qemu patch
> > I posted separately, on host):
> >
> >
> > diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
> > index a6983b2..47f9eff 100644
> > --- a/arch/x86/include/asm/bitops.h
> > +++ b/arch/x86/include/asm/bitops.h
> > @@ -28,11 +28,13 @@
> > #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)
>
> What's this doing here?

Ugh. More leftovers from when I had inline asm here.
Will remove.

> >
> > +/* size alignment is implied but just to make it explicit. */
> > +static DEFINE_PER_CPU(unsigned long, kvm_apic_eoi) __aligned(2) =
> > + KVM_PV_EOI_DISABLED;
>
> You're actually breaking the alignment. ulong has 8 byte alignment
> sometimes and you can make it cross cache boundary this way.

No, if you look at the definition of __aligned
you will see that it limits the alignment from below.
Compiler still applies the natural size alignment.
You are not the first to get confused. So I wonder: is it better
to add a comment or simply remove __aligned here.

> > +
> > void __cpuinit kvm_guest_cpu_init(void)
> > {
> > if (!kvm_para_available())
> > @@ -300,11 +320,17 @@ void __cpuinit kvm_guest_cpu_init(void)
> > smp_processor_id());
> > }
> >
> > + if (kvm_para_has_feature(KVM_FEATURE_PV_EOI)) {
> > + __get_cpu_var(kvm_apic_eoi) = 0;
> > + wrmsrl(MSR_KVM_PV_EOI_EN, __pa(&__get_cpu_var(kvm_apic_eoi)) |
> > + KVM_MSR_ENABLED);
>
> Bad formatting.

I guess temporary will make it prettier.
unsigned long pa;
__get_cpu_var(kvm_apic_eoi) = 0;
pa = __pa(&__get_cpu_var(kvm_apic_eoi)) | KVM_MSR_ENABLED;
wrmsrl(MSR_KVM_PV_EOI_EN, pa);

or did I miss the point?

> > + }
> > +
> > if (has_steal_clock)
> > kvm_register_steal_time();
> > }
> >
>
>
> Please check that the kexec path also disables pveoi.

The chunk in kvm_pv_guest_cpu_reboot does this, doesn't it?

> --
> error compiling committee.c: too many arguments to function
>