From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jens Axboe <axboe@kernel.dk>
Subject: Re: virtio-rng only returns zeros with CONFIG_HW_RANDOM=m
Date: Thu, 30 May 2013 09:18:40 +0200
Message-ID: <20130530071840.GR22724@kernel.dk>
References: <20130224231154.GA30477@hall.aurel32.net>
 <8738wifwb2.fsf@rustcorp.com.au>
 <20130227132213.GF4615@kernel.dk>
 <874ndlmg0h.fsf@rustcorp.com.au>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Cc: Aurelien Jarno <aurelien@aurel32.net>, kvm@vger.kernel.org,
	Ian Molton <ian.molton@collabora.co.uk>,
	Matt Mackall <mpm@selenic.com>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Ben Hutchings <ben@decadent.org.uk>
To: Rusty Russell <rusty@rustcorp.com.au>
Return-path: <kvm-owner@vger.kernel.org>
Received: from merlin.infradead.org ([205.233.59.134]:40310 "EHLO
	merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S967578Ab3E3HTb (ORCPT <rfc822;kvm@vger.kernel.org>);
	Thu, 30 May 2013 03:19:31 -0400
Content-Disposition: inline
In-Reply-To: <874ndlmg0h.fsf@rustcorp.com.au>
Sender: kvm-owner@vger.kernel.org
List-ID: <kvm.vger.kernel.org>

On Thu, May 30 2013, Rusty Russell wrote:
> Jens Axboe <axboe@kernel.dk> writes:
> > On Wed, Feb 27 2013, Rusty Russell wrote:
> >> Aurelien Jarno <aurelien@aurel32.net> writes:
> >> > Hi,
> >> >
> >> > I have noticed that virtio-rng only returns zero for kernels >= 2.6.33
> >> > built with CONFIG_HW_RANDOM=m. This is a bit much too predictable for a
> >> > random generator ;-).
> >> 
> >> Wow.  Fortunately, all of SLES, RHEL, Ubuntu or Fedora set
> >> CONFIG_HW_RANDOM=y.  What do they know that we don't?
> >> 
> >> Oops, looks like Debian testing: config-3.2.0-4-amd64:CONFIG_HW_RANDOM=m
> >> 
> >> > The reason for that is virtio expects guest real addresses, while
> >> > rng_core.ko (ie when built as a module) is passing a vmalloced buffer 
> >> > to the virtio-rng read function, declared as such:
> >> >
> >> >   static u8 rng_buffer[SMP_CACHE_BYTES < 32 ? 32 : SMP_CACHE_BYTES]
> >> >           __cacheline_aligned;
> >> 
> >> Yuck...  It would be nice if this has oopsed.  Jens, what about this patch?
> >> 
> >> Cheers,
> >> Rusty.
> >> 
> >> Subject: scatterlist: sg_set_buf() argument must be in linear mapping.
> >> 
> >> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> >> 
> >> diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
> >> index 4bd6c06..9365375 100644
> >> --- a/include/linux/scatterlist.h
> >> +++ b/include/linux/scatterlist.h
> >> @@ -111,6 +111,9 @@ static inline struct page *sg_page(struct scatterlist *sg)
> >>  static inline void sg_set_buf(struct scatterlist *sg, const void *buf,
> >>  			      unsigned int buflen)
> >>  {
> >> +#ifdef CONFIG_DEBUG_SG
> >> +	BUG_ON(!virt_addr_valid(buf));
> >> +#endif
> >>  	sg_set_page(sg, virt_to_page(buf), buflen, offset_in_page(buf));
> >>  }
> >
> > Looks good to me, in lieu of being able to return an error. Want me to
> > queue it up?
> >
> > -- 
> > Jens Axboe
> 
> Ping?  I haven't seen this go into Linus' tree...

I forget if I didn't get a reply, or whether it just got lost! Queued up
now, at least.

-- 
Jens Axboe