From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:41755) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dJ0Of-0000YJ-De for qemu-devel@nongnu.org; Thu, 08 Jun 2017 12:34:58 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dJ0Ob-0000GM-Vl for qemu-devel@nongnu.org; Thu, 08 Jun 2017 12:34:57 -0400 Received: from mx1.redhat.com ([209.132.183.28]:50850) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dJ0Ob-0000Fm-PK for qemu-devel@nongnu.org; Thu, 08 Jun 2017 12:34:53 -0400 Received: from int-mx11.intmail.prod.int.phx2.redhat.com (int-mx11.intmail.prod.int.phx2.redhat.com [10.5.11.24]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 7834C43A47 for ; Thu, 8 Jun 2017 16:34:52 +0000 (UTC) References: <20170608161013.17920-1-lersek@redhat.com> From: Paolo Bonzini Message-ID: <961e1ac9-56a5-df24-9ed7-e1981e61942a@redhat.com> Date: Thu, 8 Jun 2017 18:34:49 +0200 MIME-Version: 1.0 In-Reply-To: <20170608161013.17920-1-lersek@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH] q35/mch: implement extended TSEG sizes List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Laszlo Ersek , qemu devel list Cc: "Michael S. Tsirkin" , Gerd Hoffmann On 08/06/2017 18:10, Laszlo Ersek wrote: > When the guest writes value 0xffff to this register, the value that can be > read back is that of "mch.extended-tseg-mbytes" -- unless it remains > 0xffff. The guest is required to write 0xffff first (as opposed to a > read-only register) because PCI config space is generally not cleared on > QEMU reset, and after S3 resume or reboot, new guest firmware running on > old QEMU could read a guest OS-injected value from this register. I guess that's also a reason not to make it readonly (that is, it would require some firmware code anyway to test for "readonlyness" and distinguish old machine types from new)? Thanks, Paolo