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([2001:b07:6468:f312:3473:41cf:8b30:f2e1]) by smtp.googlemail.com with ESMTPSA id j8sm19509377wmd.0.2019.01.07.08.20.35 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 07 Jan 2019 08:20:36 -0800 (PST) Subject: Re: [PATCH 7/11] KVM: Remove redundant check in the kvm_get_dirty_log_protect() To: Sean Christopherson , lantianyu1986@gmail.com Cc: kvm@vger.kernel.org, rkrcmar@redhat.com, catalin.marinas@arm.com, will.deacon@arm.com, linux-kernel@vger.kernel.org, paulus@ozlabs.org, hpa@zytor.com, kys@microsoft.com, kvmarm@lists.cs.columbia.edu, mpe@ellerman.id.au, x86@kernel.org, linux@armlinux.org.uk, michael.h.kelley@microsoft.com, mingo@redhat.com, benh@kernel.crashing.org, jhogan@kernel.org, linux-mips@vger.kernel.org, Lan Tianyu , marc.zyngier@arm.com, kvm-ppc@vger.kernel.org, bp@alien8.de, tglx@linutronix.de, linux-arm-kernel@lists.infradead.org, christoffer.dall@arm.com, ralf@linux-mips.org, paul.burton@mips.com, vkuznets@redhat.com, linuxppc-dev@lists.ozlabs.org References: <20190104085405.40356-1-Tianyu.Lan@microsoft.com> <20190104085405.40356-8-Tianyu.Lan@microsoft.com> <20190104155036.GA11288@linux.intel.com> From: Paolo Bonzini Openpgp: preference=signencrypt Autocrypt: addr=pbonzini@redhat.com; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.1 MIME-Version: 1.0 In-Reply-To: <20190104155036.GA11288@linux.intel.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-mips-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-mips@vger.kernel.org On 04/01/19 16:50, Sean Christopherson wrote: > Tangentially related, does mmu_lock actually need to be held while we > walk dirty_bitmap in kvm_{clear,get}_dirty_log_protect()? The bitmap > itself is protected by slots_lock (a lockdep assertion would be nice > too), e.g. can we grab the lock iff dirty_bitmap[i] != 0? Yes, we could avoid grabbing it as long as the bitmap is zero. However, without kvm->manual_dirty_log_protect, the granularity of kvm_get_dirty_log_protect() is too coarse so it won't happen in practice. Instead, with the new manual clear, kvm_get_dirty_log_protect() does not take the lock and a well-written userspace is not going to call the clear ioctl unless some bits are set. Paolo