From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41318) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Wx2Hv-0006EA-Do for qemu-devel@nongnu.org; Tue, 17 Jun 2014 18:55:40 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1Wx2Hn-0004xG-Nj for qemu-devel@nongnu.org; Tue, 17 Jun 2014 18:55:35 -0400 Received: from gate.crashing.org ([63.228.1.57]:48409) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1Wx2Hn-0004xA-F1 for qemu-devel@nongnu.org; Tue, 17 Jun 2014 18:55:27 -0400 Message-ID: <1403045714.7661.179.camel@pasglop> From: Benjamin Herrenschmidt Date: Wed, 18 Jun 2014 08:55:14 +1000 In-Reply-To: References: <1402974463.7661.102.camel@pasglop> <1403001900.1614.10.camel@nilsson.home.kraxel.org> <1403003721.7661.148.camel@pasglop> <1403006267.1614.14.camel@nilsson.home.kraxel.org> <1403040734.7661.173.camel@pasglop> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [RFC] qemu VGA endian swap low level drawing changes List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Maydell Cc: Alexey Kardashevskiy , Paolo Bonzini , Alexander Graf , Gerd Hoffmann , "qemu-devel@nongnu.org" On Tue, 2014-06-17 at 23:12 +0100, Peter Maydell wrote: > On 17 June 2014 22:32, Benjamin Herrenschmidt wrote: > > Additionally, I wouldn't mind of we did a quick "trick" equivalent (but > > cleaner) to what I did in my patch which is when the pseries guest calls > > the hypervisor call to change the interrupt endian mode, we notify VGA > > and switch its endian mode, so we work "by default" with kernels not > > updated to know about that register. But this is open for debate. It's > > somewhat "acceptable" in the context of our hypercall being a > > "paravirtualized" interface, so it can be argued that the hypercall > > poking at the VGA chip is equivalent to some FW doing so :-) > > I'm definitely against this. Have your guest change the behaviour > of the VGA device by explicitly prodding the VGA device, not by > back-door side-effects of something else that it happens to do > on one particular guest for one particular architecture, please. But that means modifying guests ... obviously you live in a world where you don't have to live with existing enterprise distros backward compatibility :-) I understand the reluctance against backdoor side effects in general, but as I said, this is a hypervisor call that basically says "change system endianness". It does make some amount of sense to have in that case the hypervisor (which here is qemu) go adjust the endianness setting in some devices as well as the cores. Cheers, Ben.