linux-kernel - Re: [PATCH] remove static declaration from wall clock version

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Date:	Sun, 08 Mar 2009 14:15:08 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Glauber Costa <glommer@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] remove static declaration from wall clock version

Glauber Costa wrote:
> Matt T. Yourst noted that we're currently having a dumb
> race for no reason in paravirtual wall clock. This is due
> to the use of a static variable to hold the counting.
>
> This can race with multiple guests reading wallclock
> at the same time, since the static variable value would
> then be accessible to all callers. This wasn't noted
> before because it is a rather rare scenario.
>
> Instead, just use a normal stack variable. This will
> mean that each caller will have it's version written
> separatedly. No need for a global counter.
>
> Signed-off-by: Glauber Costa <glommer@...hat.com>
> ---
>  arch/x86/kvm/x86.c |    4 +---
>  1 files changed, 1 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 2511708..d7236f6 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -548,15 +548,13 @@ static int do_set_msr(struct kvm_vcpu *vcpu, unsigned index, u64 *data)
>  
>  static void kvm_write_wall_clock(struct kvm *kvm, gpa_t wall_clock)
>  {
> -	static int version;
> +	int version = 1;
>  	struct pvclock_wall_clock wc;
>  	struct timespec now, sys, boot;
>  
>  	if (!wall_clock)
>  		return;
>  
> -	version++;
> -
>  	kvm_write_guest(kvm, wall_clock, &version, sizeof(version));
>  

Suppose currently version == 2.

guest: read version (2)
guest: read sec
    host: write version (1)
    host: write sec
    host: write nsec
    host: write version (2)
guest: read nsec
guest: read version (2)

So now we have inconsistent time (sec from old data, nsec from new data).

We need to make version a per-vm value.  Best to read it from guest 
memory, so nothing special needs to be done for live migration.  Also 
use mutual exclusion in kvm_write_wall_clock() - sequence locks don't 
support multiple writers.

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

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