Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752505AbbGIGng (ORCPT ); Thu, 9 Jul 2015 02:43:36 -0400 Received: from mx1.redhat.com ([209.132.183.28]:43401 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751237AbbGIGn3 (ORCPT ); Thu, 9 Jul 2015 02:43:29 -0400 Message-ID: <559E180E.8080308@redhat.com> Date: Thu, 09 Jul 2015 08:43:26 +0200 From: Laszlo Ersek User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: Paolo Bonzini , Bandan Das , kvm@vger.kernel.org CC: linux-kernel@vger.kernel.org, qemu-devel@nongnu.org Subject: Re: [PATCH] KVM: x86: Add host physical address width capability References: <559E101A.7080601@redhat.com> In-Reply-To: <559E101A.7080601@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: 2669 Lines: 78 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.) Thanks Laszlo > > Paolo > >> case KVM_CAP_PV_MMU: /* obsolete */ >> r = 0; >> break; >> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h >> index 716ad4a..e7949a1 100644 >> --- a/include/uapi/linux/kvm.h >> +++ b/include/uapi/linux/kvm.h >> @@ -817,6 +817,7 @@ struct kvm_ppc_smmu_info { >> #define KVM_CAP_DISABLE_QUIRKS 116 >> #define KVM_CAP_X86_SMM 117 >> #define KVM_CAP_MULTI_ADDRESS_SPACE 118 >> +#define KVM_CAP_PHY_ADDR_WIDTH 119 >> >> #ifdef KVM_CAP_IRQ_ROUTING >> >> -- 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/