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

Avi Kivity
2012-06-18T14:17:43Z

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?

>
> +/* 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.

> +
> 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.

> + }
> +
> if (has_steal_clock)
> kvm_register_steal_time();
> }
>

Please check that the kexec path also disables pveoi.

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