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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id 90sm11387479edr.69.2021.03.10.09.44.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 10 Mar 2021 09:44:29 -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> <2749fe68-acbb-8f4d-dc76-4cb23edb9b35@redhat.com> <871rcmhq43.wl-maz@kernel.org> From: Paolo Bonzini Message-ID: Date: Wed, 10 Mar 2021 18:44:27 +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: <871rcmhq43.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: kvm@vger.kernel.org On 10/03/21 18:31, Marc Zyngier wrote: >> 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. > Which is why we have per-cpu counters already. Making use of them > doesn't seem that outlandish. But you wouldn't be able to guarantee consistency anyway, would you? You *could* copy N*M counters to userspace, but there's no guarantee that they are consistent, neither within a single vCPU nor within a single counter. >> 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. > > Having that baseline in the cover letter would be a good start, as > well as an indication of the frequency this is used at. Can't disagree, especially on the latter which I have no idea about. Paolo