From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marcelo Tosatti <mtosatti@redhat.com>
Subject: Re: [patch 10/16] x86: vdso: pvclock gettime support
Date: Thu, 1 Nov 2012 19:42:43 -0200
Message-ID: <20121101214243.GC19712@amt.cnet>
References: <20121031224656.417434866@redhat.com>
 <20121031224824.293748067@redhat.com>
 <50928A2A.70104@parallels.com>
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Cc: kvm@vger.kernel.org, johnstul@us.ibm.com, jeremy@goop.org,
	zamsden@gmail.com, gleb@redhat.com, avi@redhat.com,
	pbonzini@redhat.com
To: Glauber Costa <glommer@parallels.com>
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On Thu, Nov 01, 2012 at 06:41:46PM +0400, Glauber Costa wrote:
> On 11/01/2012 02:47 AM, Marcelo Tosatti wrote:
> > +#ifdef CONFIG_PARAVIRT_CLOCK
> > +
> > +static notrace const struct pvclock_vsyscall_time_info *get_pvti(int cpu)
> > +{
> > +	const aligned_pvti_t *pvti_base;
> > +	int idx = cpu / (PAGE_SIZE/PVTI_SIZE);
> > +	int offset = cpu % (PAGE_SIZE/PVTI_SIZE);
> > +
> > +	BUG_ON(PVCLOCK_FIXMAP_BEGIN + idx > PVCLOCK_FIXMAP_END);
> > +
> > +	pvti_base = (aligned_pvti_t *)__fix_to_virt(PVCLOCK_FIXMAP_BEGIN+idx);
> > +
> > +	return &pvti_base[offset].info;
> > +}
> > +
> Does BUG_ON() really do what you believe it does while in userspace
> context? We're not running with the kernel descriptors, so this will
> probably just kill the process without any explanation

A coredump is generated which can be used to trace back to ud2a instruction
at the vdso code.