linux-kernel - Re: [PATCH 2/6 V2] Add functions to check if the host has stopped the vm

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Date:	Tue, 01 Nov 2011 14:28:31 -0500
From:	Anthony Liguori <aliguori@...ibm.com>
To:	Eric B Munson <emunson@...bm.net>
CC:	avi@...hat.com, mingo@...hat.com, x86@...nel.org, hpa@...or.com,
	arnd@...db.de, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	linux-arch@...r.kernel.org, ryanh@...ux.vnet.ibm.com
Subject: Re: [PATCH 2/6 V2] Add functions to check if the host has stopped
 the vm

On 10/31/2011 03:07 PM, Eric B Munson wrote:
> When a host stops or suspends a VM it will set a flag to show this.  The
> watchdog will use these functions to determine if a softlockup is real, or the
> result of a suspended VM.
>
> Signed-off-by: Eric B Munson<emunson@...bm.net>
> ---
>   arch/x86/include/asm/pvclock.h |    2 ++
>   arch/x86/kernel/kvmclock.c     |   19 +++++++++++++++++++
>   2 files changed, 21 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/include/asm/pvclock.h b/arch/x86/include/asm/pvclock.h
> index c59cc97..7d3ba41 100644
> --- a/arch/x86/include/asm/pvclock.h
> +++ b/arch/x86/include/asm/pvclock.h
> @@ -13,6 +13,8 @@ void pvclock_read_wallclock(struct pvclock_wall_clock *wall,
>   			    struct timespec *ts);
>   void pvclock_resume(void);
>
> +bool kvm_check_and_clear_host_stopped(int cpu);

A kvm-specific function shouldn't be declared in pvclock.h as this is shared 
code between KVM and Xen.

> +
>   /*
>    * Scale a 64-bit delta by scaling and multiplying by a 32-bit fraction,
>    * yielding a 64-bit result.
> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> index c1a0188..8ddcfaf 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/arch/x86/kernel/kvmclock.c
> @@ -113,6 +113,25 @@ static void kvm_get_preset_lpj(void)
>   	preset_lpj = lpj;
>   }
>
> +bool kvm_check_and_clear_host_stopped(int cpu)
> +{
> +	bool ret = false;
> +	struct pvclock_vcpu_time_info *src;
> +
> +	/*
> +	 * per_cpu() is safe here because this function is only called from
> +	 * timer functions where preemption is already disabled.
> +	 */
> +	WARN_ON(!in_atomic());
> +	src =&per_cpu(hv_clock, cpu);
> +	if ((src->flags&  PVCLOCK_GUEST_STOPPED) != 0) {
> +		src->flags = src->flags&  (~PVCLOCK_GUEST_STOPPED);
> +		ret = true;
> +	}

It would be helpful to separate out the patches for guest enablement from host 
enablement.  This is a guest function, correct?

Don't you need an atomic getandset here?  Can the host clear this flag or only 
set it?

It's ashame we don't have a proper pvclock spec in Documentation/.  I would 
suggest at least adding a description of how this bit works in the first patch.

Regards,

Anthony Liguori

> +
> +	return ret;
> +}
> +
>   static struct clocksource kvm_clock = {
>   	.name = "kvm-clock",
>   	.read = kvm_clock_get_cycles,

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