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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id k9sm8884545edn.68.2021.03.10.08.03.43 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 10 Mar 2021 08:03:44 -0800 (PST) Subject: Re: [RFC PATCH 3/4] KVM: stats: Add ioctl commands to pull statistics in binary format To: Marc Zyngier , Jing Zhang Cc: KVM , KVM ARM , Linux MIPS , KVM PPC , Linux S390 , Linux kselftest , James Morse , Julien Thierry , Suzuki K Poulose , Will Deacon , Huacai Chen , Aleksandar Markovic , Thomas Bogendoerfer , Paul Mackerras , Christian Borntraeger , Janosch Frank , David Hildenbrand , Cornelia Huck , Claudio Imbrenda , Sean Christopherson , Vitaly Kuznetsov , Jim Mattson , Peter Shier , Oliver Upton , David Rientjes , Emanuele Giuseppe Esposito References: <20210310003024.2026253-1-jingzhangos@google.com> <20210310003024.2026253-4-jingzhangos@google.com> <875z1zxb11.wl-maz@kernel.org> From: Paolo Bonzini Message-ID: Date: Wed, 10 Mar 2021 17:03:42 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: <875z1zxb11.wl-maz@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-s390@vger.kernel.org On 10/03/21 16:51, Marc Zyngier wrote: >> + kvm_for_each_vcpu(j, vcpu, kvm) { >> + pdata = data + VM_STAT_COUNT; >> + for (i = 0; i < VCPU_STAT_COUNT; i++, pdata++) >> + *pdata += *((u64 *)&vcpu->stat + i); > Do you really need the in-kernel copy? Why not directly organise the > data structures in a way that would allow a bulk copy using > copy_to_user()? The result is built by summing per-vCPU counters, so that the counter updates are fast and do not require a lock. So consistency basically cannot be guaranteed. Paolo