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([2001:b07:6468:f312:3c1c:ffba:c624:29b8]) by smtp.gmail.com with ESMTPSA id f4sm3124661ejk.17.2020.05.27.07.54.37 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 27 May 2020 07:54:38 -0700 (PDT) Subject: Re: [PATCH v3 14/22] microvm: use 2G split unconditionally To: Igor Mammedov References: <20200520132003.9492-1-kraxel@redhat.com> <20200520132003.9492-15-kraxel@redhat.com> <20200521112921.641f12bb@redhat.com> <20200525114508.hhvyzjiqja6knz6l@sirius.home.kraxel.org> <20200525183615.5f8e67d3@redhat.com> <20200526044839.bncj6iltugnzgfmy@sirius.home.kraxel.org> <20200527142545.32e91049@redhat.com> <58ced517-1ad4-c895-afe6-fec690791f4f@redhat.com> <20200527162646.694b2d47@redhat.com> From: Paolo Bonzini Message-ID: Date: Wed, 27 May 2020 16:54:34 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <20200527162646.694b2d47@redhat.com> Content-Language: en-US X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=205.139.110.61; envelope-from=pbonzini@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/05/27 05:46:19 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_PASS=-0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Peter Maydell , Eduardo Habkost , Sergio Lopez , "Michael S. Tsirkin" , qemu-devel@nongnu.org, Shannon Zhao , qemu-arm@nongnu.org, Gerd Hoffmann , Richard Henderson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 27/05/20 16:26, Igor Mammedov wrote: > On Wed, 27 May 2020 15:06:28 +0200 > Paolo Bonzini wrote: > >> On 27/05/20 14:25, Igor Mammedov wrote: >>>> (2) we loose a gigabyte page. >>> I'm not sure waht exactly we loose in this case. >>> Lets assume we allocating guest 5G of continuous RAM using 1G huge pages, >>> in this case I'd think that on host side MMIO overlay won't affect RAM blob >>> on guest side pagetables will be fragmented due to MMIO holes, but guest still >>> could use huge pages smaller ones in fragmented area and 1G where there is no fragmentation. >> >> Access to the 3G-4G area would not be able to use 1G EPT pages. > Could it use 2Mb pages instead of 1Gb? Yes, probably a mix of 2 MiB pages and 4 KiB pages around the memslot splits. > Do we really care about 1 gigabyte huge page in microvm intended usecase? > (fast starting VMs for microservices like FaaS, which unlikely would use much memory to begin with) I honestly don't think it's measurable, but at least in theory we care because such workloads could have more TLB misses (relative to the execution time) than long-lasting VMs. Paolo