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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id j5sm969210edl.42.2021.02.03.06.33.39 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 03 Feb 2021 06:33:40 -0800 (PST) Subject: Re: [PATCH 2/2] KVM: Scalable memslots implementation To: David Hildenbrand , "Maciej S. Szmigiero" , Sean Christopherson Cc: Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Igor Mammedov , Marc Zyngier , James Morse , Julien Thierry , Suzuki K Poulose , Huacai Chen , Aleksandar Markovic , Paul Mackerras , Christian Borntraeger , Janosch Frank , Cornelia Huck , Claudio Imbrenda , Joerg Roedel , kvm@vger.kernel.org, linux-kernel@vger.kernel.org References: <4d748e0fd50bac68ece6952129aed319502b6853.1612140117.git.maciej.szmigiero@oracle.com> <9e6ca093-35c3-7cca-443b-9f635df4891d@maciej.szmigiero.name> <4bdcb44c-c35d-45b2-c0c1-e857e0fd383e@redhat.com> <5efd931f-9d69-2936-89e8-278fe106616d@redhat.com> <307603f3-52a8-7464-ba98-06cbe4ddd408@redhat.com> From: Paolo Bonzini Message-ID: Date: Wed, 3 Feb 2021 15:33:38 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03/02/21 14:52, David Hildenbrand wrote: >> >> However, note that the TDP MMU does not need an rmap at all.  Since that >> one is getting ready to become the default, the benefits of working on >> the rmap would be quite small and only affect nested virtualization. > > Right, but we currently always have to allocate it. > > 8 bytes per 4k page, 8 bytes per 2M page, 8 bytes per 1G page. > > The 4k part alone is 0.2% of the memblock size. > > For a 1 TB memslot we might "waste" > 2 GB on rmap arrays. > > (that's why I am asking :) ) Yes, we're in violent agreement. :) I mean that the simplest solution would be to allocate it lazily when building a non-TDP root. Paolo