From mboxrd@z Thu Jan 1 00:00:00 1970 From: Laszlo Ersek Subject: Re: [PATCH] KVM: x86: Add host physical address width capability Date: Fri, 10 Jul 2015 17:06:55 +0200 Message-ID: <559FDF8F.1020109@redhat.com> References: <559E101A.7080601@redhat.com> <559E180E.8080308@redhat.com> <559E6BE5.4030000@redhat.com> <559EC3FC.8050204@redhat.com> <559FD30C.4000209@redhat.com> <559FDD44.1020008@redhat.com> <559FDDC7.3060306@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, qemu-devel@nongnu.org To: Paolo Bonzini , Bandan Das Return-path: In-Reply-To: <559FDDC7.3060306@redhat.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 List-Id: kvm.vger.kernel.org On 07/10/15 16:59, Paolo Bonzini wrote: > > > On 10/07/2015 16:57, Laszlo Ersek wrote: >>>> ... In any case, please understand that I'm not campaigning for this >>>> warning :) IIRC the warning was your (very welcome!) idea after I >>>> reported the problem; I'm just trying to ensure that the warning match >>>> the exact issue I encountered. >>> >>> Yup. I think the right thing to do would be to hide memory above the >>> limit. >> How so? >> >> - The stack would not be doing what the user asks for. Pass -m , >> and the guest would silently see less memory. If the user found out, >> he'd immediately ask (or set out debugging) why. I think if the user's >> request cannot be satisfied, the stack should fail hard. > > That's another possibility. I think both of them are wrong depending on > _why_ you're using "-m " in the first place. > > Considering that this really happens (on Xeons) only for 1TB+ guests, I reported this issue because I ran into it with a ~64GB guest. From my /proc/cpuinfo: model name : Intel(R) Core(TM) i7 CPU M 620 @ 2.67GHz address sizes : 36 bits physical, 48 bits virtual I was specifically developing 64GB+ support for OVMF, and this limitation caused me to think that there was a bug in my OVMF patches. (There wasn't.) An error message from QEMU, advising me to turn off EPT, would have saved me many hours. Thanks Laszlo > it's probably just for debugging and then hiding the memory makes some > sense. > > Paolo >