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([2001:b07:6468:f312:48f9:bea:a04c:3dfe]) by smtp.googlemail.com with ESMTPSA id jg32sm1900192ejc.43.2021.12.10.08.13.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 10 Dec 2021 08:13:38 -0800 (PST) Sender: Paolo Bonzini Message-ID: <8ab8833f-2a89-71ff-98da-2cfbb251736f@redhat.com> Date: Fri, 10 Dec 2021 17:13:29 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0 Subject: Re: [PATCH 1/7] KVM: x86: Retry page fault if MMU reload is pending and root has no sp Content-Language: en-US To: Sean Christopherson Cc: Christian Borntraeger , Janosch Frank , David Hildenbrand , Claudio Imbrenda , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Maxim Levitsky , Ben Gardon , Lai Jiangshan References: <20211209060552.2956723-1-seanjc@google.com> <20211209060552.2956723-2-seanjc@google.com> From: Paolo Bonzini In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/10/21 17:01, Sean Christopherson wrote: >> KVM_REQ_MMU_RELOAD is raised after kvm->arch.mmu_valid_gen is fixed (of >> course, otherwise the other CPU might just not see any obsoleted page >> from the legacy MMU), therefore any check on KVM_REQ_MMU_RELOAD is just >> advisory. > > I disagree. IMO, KVM should not be installing SPTEs into obsolete shadow pages, > which is what continuing on allows. I don't _think_ it's problematic, but I do > think it's wrong. > > [...] Eh, for all intents and purposes, KVM_REQ_MMU_RELOAD very much says > special roots are obsolete. The root will be unloaded, i.e. will no > longer be used, i.e. is obsolete. I understand that---but it takes some unspoken details to understand that. In particular that both kvm_reload_remote_mmus and is_page_fault_stale are called under mmu_lock write-lock, and that there's no unlock between updating mmu_valid_gen and calling kvm_reload_remote_mmus. (This also suggests, for the other six patches, keeping kvm_reload_remote_mmus and just moving it to arch/x86/kvm/mmu/mmu.c, with an assertion that the MMU lock is held for write). But since we have a way forward for having no special roots to worry about, it seems an unnecessary overload for 1) a patch that will last one or two releasees at most 2) a case that has been handled in the inefficient way forever. Paolo > The other way to check for an invalid special root would be to treat > it as obsolete if any of its children in entries 0-3 are present and > obsolete. That would be more precise, but it provides no benefit > given KVM's current implementation. > > I'm not completely opposed to doing nothing, but I do think it's > silly to continue on knowing that the work done by the page fault is > all but gauranteed to be useless. >