From mboxrd@z Thu Jan 1 00:00:00 1970 From: Amit Shah Subject: Re: Extending virtio_console to support multiple ports Date: Thu, 27 Aug 2009 12:22:51 +0530 Message-ID: <20090827065251.GB3218@amit-x200.redhat.com> References: <1251181044-3696-1-git-send-email-amit.shah@redhat.com> <20090826112718.GA11117@amit-x200.redhat.com> <20090826154552.GA31910@amit-x200.redhat.com> <1251349485.6267.6.camel@concordia> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org, linuxppc-dev@ozlabs.org, linux-kernel@vger.kernel.org, miltonm@bga.com, qemu-devel@nongnu.org, borntraeger@de.ibm.com, brueckner@linux.vnet.ibm.com, virtualization@lists.linux-foundation.org, alan@linux.intel.com To: Michael Ellerman Return-path: Content-Disposition: inline In-Reply-To: <1251349485.6267.6.camel@concordia> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: linuxppc-dev-bounces+glppd-linuxppc64-dev=m.gmane.org@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+glppd-linuxppc64-dev=m.gmane.org@lists.ozlabs.org List-Id: kvm.vger.kernel.org On (Thu) Aug 27 2009 [15:04:45], Michael Ellerman wrote: > 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 ;) > > http://git.kernel.org/?p=linux/kernel/git/torvalds/old-2.6-bkcvs.git;a=commitdiff;h=d450b4ae023fb4be175389c18f4f87677da03020 Thanks for the link! (and this general area might be the one that doesn't get major upheavals in 5-yr spans :-) Amit