Re: [Xen-devel] The hypercall will fail and return EFAULT when the page becomes COW by forking process in linux

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2012-07-25 at 12:48 +0100, Wangzhenguo wrote:
> > From: Ian Campbell [mailto:Ian.Campbell@xxxxxxxxxx]
> > Sent: Tuesday, July 24, 2012 9:06 PM
> > 
> > Hrm. yes. This basically harks back to the use of mlock as a surrogate
> > for the requirement to properly lock down memory for use as a hypercall
> > argument. This is flawed in ways other than this (i.e. NUMA memory
> > migration would also break it).
> > 
> > The correct answer here is a special device (e.g. /dev/xen/hypercall?)
> > which can be mmap by libxc to provide memory specifically for this
> > purspoe which is fully locked down (e.g. VM_DONTCOPY and whatever else
> > is required).
> 
> Thanks for your reply. I see the madvise syscall can make the vma to
> be VM_DONTCOPY by deliverying MADV_DONTFORK advice.

I didn't know about this MADV option, it sounds like the right idea to
me.

>  I'll fixup and test it.

> > 
> > The libxc "osdep" interface (see xenctrlosdep.h) provides a framework
> > for doing this, it just doesn't actually implement the use of the
> > special driver yet.
> > 
> > Is that something you might be interested in coding up?
> > 
> > Ian.
> 



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