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[Qemu-devel] Re: Extending virtio_console to support multiple ports

From: Michael Ellerman
Subject: [Qemu-devel] Re: Extending virtio_console to support multiple ports
Date: Thu, 27 Aug 2009 15:04:45 +1000

On Wed, 2009-08-26 at 21:15 +0530, Amit Shah wrote:
> [cc'ing some people who have made some commits in hvc_console.c]
> On (Wed) Aug 26 2009 [16:57:18], Amit Shah wrote:
> > On (Tue) Aug 25 2009 [11:47:20], Amit Shah wrote:
> > > 
> > > Hello all,
> > > 
> > > Here is a new iteration of the patch series that implements a
> > > transport for guest and host communications.
> > > 
> > > The code has been updated to reuse the virtio-console device instead
> > > of creating a new virtio-serial device.
> > 
> > And the problem now is that hvc calls the put_chars function with
> > spinlocks held and we now allocate pages in send_buf(), called from
> > put_chars.
> > 
> > A few solutions:
> [snip]
> > - Convert hvc's usage of spinlocks to mutexes. I've no idea how this
> >   will play out; I'm no expert here. But I did try doing this and so far
> >   it all looks OK. No lockups, lockdep warnings, nothing. I have full
> >   debugging enabled. But this doesn't mean it's right.
> So just to test this further I added the capability to have more than
> one hvc console spawn from virtio_console, created two consoles and did
> a 'cat' of a file in each of the virtio-consoles. It's been running for
> half an hour now without any badness. No spew in debug logs too.
> I also checked the code in hvc_console.c that takes the spin_locks.
> Nothing there that runs from (or needs to run from) interrupt context.
> So the change to mutexes does seem reasonable. Also, the spinlock code
> was added really long back -- git blame shows Linus' first git commit
> introduced them in the git history, so it's pure legacy baggage.

I won't tell Ryan you called his code "pure legacy baggage" if you
don't ;)



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