linux-kernel - Re: [PATCHv2] fix regression caused by e48672fa25e879f7ae21785c7efd187738139593

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Message-ID: <20110326144729.GA29447@nik-comp.lan>
Date:	Sat, 26 Mar 2011 15:47:29 +0100
From:	Nikola Ciprich <extmaillist@...uxbox.cz>
To:	Zachary Amsden <zamsden@...hat.com>
Cc:	KVM list <kvm@...r.kernel.org>,
	Linux kernel list <linux-kernel@...r.kernel.org>,
	nikola.ciprich@...uxbox.cz, Avi Kivity <avi@...hat.com>,
	Glauber Costa <glommer@...hat.com>
Subject: Re: [PATCHv2] fix regression caused by
	e48672fa25e879f7ae21785c7efd187738139593

> So something bothers me still about this bug.  What you did correctly  
> restores the old behavior - but it shouldn't be fixing a bug.
>
> The only reason you need to schedule an update for the KVM clock area is  
> if a new VCPU has been created, you have an unstable TSC.. or something  
> changes the VM's kvmclock offset.
>
> So this change could in fact be hiding an underlying bug - either an  
> unstable TSC is not being properly reported, the KVM clock offset is  
> being changed, we are missing a KVM clock update for secondary VCPUs -  
> or something else we don't yet understand is going on.
>
> Nikola, can you try the patch below, which reverts your change and  
> attempts to fix other possible sources of the problem, and see if it  
> still reproduces?
with Your patch, 32b SMP guests boot fine as well, without it, they
don't, so if You consider this better fix, I can acknowledge it as
working :)
n.

>
> Thanks,
>
> Zach

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 58f517b..42618fb 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2127,8 +2127,10 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  		if (check_tsc_unstable()) {
>  			kvm_x86_ops->adjust_tsc_offset(vcpu, -tsc_delta);
>  			vcpu->arch.tsc_catchup = 1;
> +			kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
>  		}
> -		kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
> +		if (vcpu->cpu == -1)
> +			kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
>  		if (vcpu->cpu != cpu)
>  			kvm_migrate_timers(vcpu);
>  		vcpu->cpu = cpu;
> @@ -3534,6 +3536,8 @@ long kvm_arch_vm_ioctl(struct file *filp,
>  		struct kvm_clock_data user_ns;
>  		u64 now_ns;
>  		s64 delta;
> +		struct kvm_vcpu *vcpu;
> +		int i;
>  
>  		r = -EFAULT;
>  		if (copy_from_user(&user_ns, argp, sizeof(user_ns)))
> @@ -3549,6 +3553,8 @@ long kvm_arch_vm_ioctl(struct file *filp,
>  		delta = user_ns.clock - now_ns;
>  		local_irq_enable();
>  		kvm->arch.kvmclock_offset = delta;
> +		kvm_for_each_vcpu(i, vcpu, kvm)
> +			kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
>  		break;
>  	}
>  	case KVM_GET_CLOCK: {


-- 
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Ing. Nikola CIPRICH
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