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[100.0.197.103]) by smtp.gmail.com with ESMTPSA id n67sm6125649qte.42.2019.05.28.19.49.57 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Tue, 28 May 2019 19:49:57 -0700 (PDT) Date: Tue, 28 May 2019 22:49:55 -0400 From: "Michael S. Tsirkin" To: Paolo Bonzini Message-ID: <20190528224810-mutt-send-email-mst@kernel.org> References: <20190528204838.21568-1-kraxel@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.85.222.194 Subject: Re: [Qemu-devel] [PATCH] q35: split memory at 2G X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Eduardo Habkost , qemu-devel@nongnu.org, Gerd Hoffmann , =?iso-8859-1?B?TOFzemzzIMlyc2Vr?= , Richard Henderson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, May 29, 2019 at 03:21:16AM +0200, Paolo Bonzini wrote: > On 28/05/19 22:48, Gerd Hoffmann wrote: > > Original q35 behavior was to split memory 2.75 GB, leaving space for the > > mmconfig bar at 0xb000000 and pci I/O window starting at 0xc0000000. > > > > Note: Those machine types have been removed from the qemu codebase > > meanwhile because they could not be live-migrated so there was little > > value in keeping them around. > > > > With the effort to allow for gigabyte-alignment of guest memory that > > behavior was changed: The split was moved to 2G, but only in case the > > memory didn't fit below 2.75 GB. > > > > So today the address space between 2G and 2,75G is not used for guest > > memory in typical use cases, where the guest memory sized at a power of > > two or a gigabyte number. But if you configure your guest with some odd > > amout of memory (such as 2.5G) the address space is used. > > Wasn't it done to ensure pre-PAE OSes could use as much memory as > possible? (If you run pre-PAE OSes with more RAM than can fit below 4G, > you can just reduce the amount of memory and get all the 2.75G). > > Paolo Absolutely. Gerd is just saying the configuration is rare enough that it's not worth worrying about. I don't know myself - why do we bother making this change? What's the advantage? -- MST