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([2001:b07:6468:f312:9541:9439:cb0f:89c]) by smtp.gmail.com with ESMTPSA id r1sm10847140wrt.73.2020.07.10.10.16.16 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 10 Jul 2020 10:16:16 -0700 (PDT) Subject: Re: [PATCH v3 0/9] KVM: Support guest MAXPHYADDR < host MAXPHYADDR To: Jim Mattson Cc: Mohammed Gamal , kvm list , LKML , Vitaly Kuznetsov , Sean Christopherson , Wanpeng Li , Joerg Roedel References: <20200710154811.418214-1-mgamal@redhat.com> <0c892b1e-6fe6-2aa7-602e-f5fadc54c257@redhat.com> From: Paolo Bonzini Message-ID: <9e784c62-15ee-63b7-4942-474493bac536@redhat.com> Date: Fri, 10 Jul 2020 19:16:14 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/07/20 19:13, Jim Mattson wrote: > On Fri, Jul 10, 2020 at 10:06 AM Paolo Bonzini wrote: >> >> On 10/07/20 18:30, Jim Mattson wrote: >>>> >>>> This can be problem when having a mixed setup of machines with 5-level page >>>> tables and machines with 4-level page tables, as live migration can change >>>> MAXPHYADDR while the guest runs, which can theoretically introduce bugs. >>> >>> Huh? Changing MAXPHYADDR while the guest runs should be illegal. Or >>> have I missed some peculiarity of LA57 that makes MAXPHYADDR a dynamic >>> CPUID information field? >> >> Changing _host_ MAXPHYADDR while the guest runs, such as if you migrate >> from a host-maxphyaddr==46 to a host-maxphyaddr==52 machine (while >> keeping guest-maxphyaddr==46). > > Ah, but what does that have to do with LA57? Intel only has MAXPHYADDR > 46 on LA57 machines (because in general OSes like to have a physical 1:1 map into the kernel part of the virtual address space, so having a higher MAXPHYADDR would be of limited use with 48-bit linear addresses). In other words, while this issue has existed forever it could be ignored until IceLake introduced MAXPHYADDR==52 machines. I'll introduce something like this in a commit message. Paolo