linux-kernel - Re: [PATCH v2 2/6] KVM-HV: KVM Steal time implementation

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Message-ID: <4D4563EB.3000203@redhat.com>
Date:	Sun, 30 Jan 2011 15:13:15 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Glauber Costa <glommer@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	aliguori@...ibm.com, Rik van Riel <riel@...hat.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 2/6] KVM-HV: KVM Steal time implementation

On 01/28/2011 09:52 PM, Glauber Costa wrote:
> To implement steal time, we need the hypervisor to pass the guest information
> about how much time was spent running other processes outside the VM.
> This is per-vcpu, and using the kvmclock structure for that is an abuse
> we decided not to make.
>
> In this patchset, I am introducing a new msr, KVM_MSR_STEAL_TIME, that
> holds the memory area address containing information about steal time
>
> This patch contains the hypervisor part for it. I am keeping it separate from
> the headers to facilitate backports to people who wants to backport the kernel
> part but not the hypervisor, or the other way around.
>
>
> @@ -1528,16 +1528,23 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 data)
>   		vcpu->arch.time_page =
>   				gfn_to_page(vcpu->kvm, data>>  PAGE_SHIFT);
>
> -		if (is_error_page(vcpu->arch.time_page)) {
> -			kvm_release_page_clean(vcpu->arch.time_page);
> -			vcpu->arch.time_page = NULL;
> -		}
>   		break;
>   	}

Unrelated?

> @@ -2106,6 +2120,25 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>   			kvm_migrate_timers(vcpu);
>   		vcpu->cpu = cpu;
>   	}
> +
> +	if (vcpu->arch.this_time_out) {
> +		u64 to = (get_kernel_ns() - vcpu->arch.this_time_out);
> +		/*
> +		 * using nanoseconds introduces noise, which accumulates easily
> +		 * leading to big steal time values. We want, however, to keep the
> +		 * interface nanosecond-based for future-proofness.
> +		 */
> +		to /= NSEC_PER_USEC;
> +		to *= NSEC_PER_USEC;

Seems there is a real problem and that this is just papering it over.  
I'd like to understand the root cause.

> +		vcpu->arch.time_out += to;
> +		kvm_write_guest(vcpu->kvm, (gpa_t)&st->steal,
> +				&vcpu->arch.time_out, sizeof(st->steal));

Error check.

> +		vcpu->arch.sversion += 2;

Doesn't survive live migration.  You need to use the version from the 
guest area.

> +		kvm_write_guest(vcpu->kvm, (gpa_t)&st->version,
> +				&vcpu->arch.sversion, sizeof(st->version));
> +		/* is it possible to have 2 loads in sequence? */
> +		vcpu->arch.this_time_out = 0;
> +	}
>   }
>

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

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