netdev - Re: [PATCHv3 2/2] vhost_net: a kernel-level virtio server

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Date:	Thu, 20 Aug 2009 17:42:56 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, mingo@...e.hu,
	linux-mm@...ck.org, akpm@...ux-foundation.org, hpa@...or.com,
	gregory.haskins@...il.com, Or Gerlitz <ogerlitz@...taire.com>
Subject: Re: [PATCHv3 2/2] vhost_net: a kernel-level virtio server

On Thu, Aug 20, 2009 at 04:31:36PM +0200, Arnd Bergmann wrote:
> On Thursday 20 August 2009, Michael S. Tsirkin wrote:
> > On Thu, Aug 20, 2009 at 03:10:54PM +0200, Arnd Bergmann wrote:
> > > On Thursday 20 August 2009, Michael S. Tsirkin wrote:
> >
> > It doesn't matter that I don't want this: allowing 1 process corrupt
> > another's memory is a security issue.  Once you get an fd, you want to
> > be able to use it without worrying that a bug in another process will
> > crash yours.
> 
> Ok, got it. Yes, that would be inacceptable.
> 
> > > > If you assume losing the code for the second error condition is OK, why
> > > > is the first one so important?  That's why I used a counter (eventfd)
> > > > per virtqueue, on error userspace can scan the ring and poll the socket
> > > > and discover what's wrong, and counter ensures we can detect that error
> > > > happened while we were not looking.
> > > 
> > > I guess we were talking about different kinds of errors here, and I'm
> > > still not sure which one you are talking about.
> > > 
> > Non fatal errors. E.g. translation errors probably should be
> > non-fatal. I can also imagine working around guest bugs in
> > userspace.
> 
> Ah, so I guess the confusion was that I was worried about
> errors coming from the socket file descriptor, while you
> were thinking of errors from the guest side, which I did not
> expect to happen.
> 
> The errors from the socket (or chardev, as that was the
> start of the argument) should still fit into the categories
> that I mentioned, either they can be handled by the host
> kernel, or they are fatal.

Hmm, are you sure? Imagine a device going away while socket is bound to
it.  You get -ENXIO. It's not fatal in a sense that you can bind the
socket to another device and go on, right?

> I'll read up in your code to see how you handle asynchronous
> non-fatal errors from the guest. Intuitively, I'd still
> assume that returning the first error should be enough
> because it will typically mean that you cannot continue
> without fixing it up first, and you might get the next
> error immediately after that.
> 
> 	Arnd <><

Yes, but need to be careful not to lose that next error, and these
errors only block one queue. I handle this by reporting them on an
eventfd, per vq, which userspace can poll. 

-- 
MST
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