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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id n25sm11384840edq.55.2021.03.10.09.11.48 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 10 Mar 2021 09:11:49 -0800 (PST) Subject: Re: [RFC PATCH 3/4] KVM: stats: Add ioctl commands to pull statistics in binary format To: Marc Zyngier Cc: Jing Zhang , 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> <8735x3x7lu.wl-maz@kernel.org> From: Paolo Bonzini Message-ID: <2749fe68-acbb-8f4d-dc76-4cb23edb9b35@redhat.com> Date: Wed, 10 Mar 2021 18:11:47 +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: <8735x3x7lu.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 18:05, Marc Zyngier wrote: > On Wed, 10 Mar 2021 16:03:42 +0000, > Paolo Bonzini wrote: >> >> 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. > > Sure, but I wonder whether there is scope for VM-global counters to be > maintained in parallel with per-vCPU counters if speed/efficiency is > of the essence (and this seems to be how it is sold in the cover > letter). Maintaining VM-global counters would require an atomic instruction and would suffer lots of cacheline bouncing even on architectures that have relaxed atomic memory operations. Speed/efficiency of retrieving statistics is important, but let's keep in mind that the baseline for comparison is hundreds of syscalls and filesystem lookups. Paolo