linux-kernel - Re: [PATCH] x86: kvmclock: zero initialize pvclock shared memory area

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Message-ID: <20130610201933.GA31409@amt.cnet>
Date:	Mon, 10 Jun 2013 17:19:33 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Igor Mammedov <imammedo@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	eabatalov89@...il.com
Subject: Re: [PATCH] x86: kvmclock: zero initialize pvclock shared memory area

On Mon, Jun 10, 2013 at 06:31:11PM +0200, Igor Mammedov wrote:
> ===
> Could be the following an acceptable fix?
> ===

Read of kvmclock should return proper value from hypervisor: system
timestamp + tsc delta.

Should find the offender site and have it register MSR_KVM_SYSTEM_TIME 
before reading the area.

> kernel might hung in pvclock_clocksource_read() due to
> uninitialized memory might contain odd version value in
> following cycle:
> 
>         do {
>                 version = __pvclock_read_cycles(src, &ret, &flags);
>         } while ((src->version & 1) || version != src->version);
> 
> if secondary kvmclock is accessed before it's registered with kvm.
> 
> Clear garbage in pvclock shared memory area right after it's
> allocated to avoid this issue.
> 
> Ref: https://bugzilla.kernel.org/show_bug.cgi?id=59521
> Signed-off-by: Igor Mammedov <imammedo@...hat.com>
> ---
>  arch/x86/kernel/kvmclock.c |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
> index d2c3812..3dd37eb 100644
> --- a/arch/x86/kernel/kvmclock.c
> +++ b/arch/x86/kernel/kvmclock.c
> @@ -242,6 +242,7 @@ void __init kvmclock_init(void)
>  	if (!mem)
>  		return;
>  	hv_clock = __va(mem);
> +	memset(hv_clock, 0, size);
>  
>  	if (kvm_register_clock("boot clock")) {
>  		hv_clock = NULL;
> -- 
> 1.7.1
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