From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: [PATCH v4 0/6] save/restore on Xen Date: Mon, 23 Jan 2012 11:18:01 -0600 Message-ID: <4F1D9649.1000102@codemonkey.ws> References: <4F19AB66.8060901@siemens.com> <4F1D4974.4090003@siemens.com> <4F1D4E43.7000501@siemens.com> <4F1D80BA.1040504@siemens.com> <4F1D9546.4040801@siemens.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4F1D9546.4040801@siemens.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+gceq-qemu-devel=gmane.org@nongnu.org Sender: qemu-devel-bounces+gceq-qemu-devel=gmane.org@nongnu.org To: Jan Kiszka Cc: Avi Kivity , "xen-devel@lists.xensource.com" , Gerd Hoffmann , "qemu-devel@nongnu.org" , Stefano Stabellini List-Id: xen-devel@lists.xenproject.org On 01/23/2012 11:13 AM, Jan Kiszka wrote: >>>> To reply to your previous question more clearly: at restore time Qemu on >>>> Xen would run in a non-standard scenario; the restore of the RAM happens >>>> before QEMU is even started. >>>> >>>> That is unfortunate but it would be very hard to change (I can give you >>>> more details if you are interested in the reasons why it would be so >>>> difficult). >>> >>> If you can't change this, you need to properly introduce this new >>> scenario - pre-initialized RAM - to the QEMU device model. Or you will >>> see breakage outside cirrus sooner or later as well. So it might be good >>> to explain the reason why it can't be changed under Xen when motivating >>> this concept extension to QEMU. >> >> OK. >> Are you thinking about introducing this concept as a new runstate? >> This special runstate could be set at restore time only on Xen. >> >> >> BTW the main reasons for having Xen saving the RAM are: >> >> - the need to support PV guests, that often run without Qemu; >> - the current save format, that is built around the fact that Xen saves the memory; >> - the fact that Qemu might be running in a very limited stub-domain. > > Your problem is not the fact that guest RAM is restored by an external > component. Your problem is that QEMU has no control over the when. If > you fix this, you could coordinate the restoring with the initial device > reset and would solve all potential current and future issues, not only > this single cirrus related one. Generally speaking, RAM is an independent device in most useful cases. Onboard RAM is a very special case because it's extremely unusual. But since some video cards can make use of dedicated external RAM, I don't think any video card really depends on initial RAM state. What's most likely here is that the VGA BIOS of a Cirrus card sets an initial RAM state during device initialization. We really should view RAM as just another device so I don't like the idea of propagating a global concept of "when RAM is restored" because that treats it specially compared to other devices. But viewing RAM as just another device, having Xen only restore a subset of devices should be a reasonable thing to do moving forward. The main problem here I believe is that we have part of the VGA Bios functionality in the hardware emulation. Regards, Anthony Liguori > > Jan >