Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754054AbbGIMlg (ORCPT ); Thu, 9 Jul 2015 08:41:36 -0400 Received: from mx1.redhat.com ([209.132.183.28]:48682 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754023AbbGIMlN (ORCPT ); Thu, 9 Jul 2015 08:41:13 -0400 Subject: Re: [PATCH] KVM: x86: Add host physical address width capability To: Laszlo Ersek , Bandan Das , kvm@vger.kernel.org References: <559E101A.7080601@redhat.com> <559E180E.8080308@redhat.com> Cc: linux-kernel@vger.kernel.org, qemu-devel@nongnu.org From: Paolo Bonzini Message-ID: <559E6BE5.4030000@redhat.com> Date: Thu, 9 Jul 2015 14:41:09 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.0.1 MIME-Version: 1.0 In-Reply-To: <559E180E.8080308@redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2487 Lines: 63 On 09/07/2015 08:43, Laszlo Ersek wrote: > On 07/09/15 08:09, Paolo Bonzini wrote: >> >> >> On 09/07/2015 00:36, Bandan Das wrote: >>> Let userspace inquire the maximum physical address width >>> of the host processors; this can be used to identify maximum >>> memory that can be assigned to the guest. >>> >>> Reported-by: Laszlo Ersek >>> Signed-off-by: Bandan Das >>> --- >>> arch/x86/kvm/x86.c | 3 +++ >>> include/uapi/linux/kvm.h | 1 + >>> 2 files changed, 4 insertions(+) >>> >>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >>> index bbaf44e..97d6746 100644 >>> --- a/arch/x86/kvm/x86.c >>> +++ b/arch/x86/kvm/x86.c >>> @@ -2683,6 +2683,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext) >>> case KVM_CAP_NR_MEMSLOTS: >>> r = KVM_USER_MEM_SLOTS; >>> break; >>> + case KVM_CAP_PHY_ADDR_WIDTH: >>> + r = boot_cpu_data.x86_phys_bits; >>> + break; >> >> Userspace can just use CPUID, can't it? > > I believe KVM's cooperation is necessary, for the following reason: > > The truncation only occurs when the guest-phys <-> host-phys translation > is done in hardware, *and* the phys bits of the host processor are > insufficient to represent the highest guest-phys address that the guest > will ever face. > > The first condition (of course) means that the truncation depends on EPT > being enabled. (I didn't test on AMD so I don't know if RVI has the same > issue.) If EPT is disabled, either because the host processor lacks it, > or because the respective kvm_intel module parameter is set so, then the > issue cannot be experienced. > > Therefore I believe a KVM patch is necessary. > > However, this specific patch doesn't seem sufficient; it should also > consider whether EPT is enabled. (And the ioctl should be perhaps > renamed to reflect that -- what QEMU needs to know is not the raw > physical address width of the host processor, but whether that width > will cause EPT to silently truncate high guest-phys addresses.) Right; if you want to consider whether EPT is enabled (which is the right thing to do, albeit it makes for a much bigger patch) a KVM patch is necessary. In that case you also need to patch the API documentation. Paolo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/