From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH v2 1/3] kvm: svm: Add support for additional SVM NPF error codes Date: Wed, 2 Aug 2017 12:42:20 +0200 Message-ID: <6afdcd42-7abe-c814-1f67-407ff91a75d2@redhat.com> References: <147992048887.27638.17559991037474542240.stgit@brijesh-build-machine> <147992049856.27638.17076562184960611399.stgit@brijesh-build-machine> <21b9f4db-f929-80f6-6ad2-6fa3b77f82c0@redhat.com> <98086274.371452.1501531542630.JavaMail.zimbra@redhat.com> <661faa8a-87af-743f-d3ea-b95ada0d7677@amd.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Cc: kvm@vger.kernel.org, thomas lendacky , rkrcmar@redhat.com, joro@8bytes.org, x86@kernel.org, linux-kernel@vger.kernel.org, mingo@redhat.com, hpa@zytor.com, tglx@linutronix.de, bp@suse.de To: Brijesh Singh Return-path: In-Reply-To: <661faa8a-87af-743f-d3ea-b95ada0d7677@amd.com> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 01/08/2017 15:36, Brijesh Singh wrote: >> >> The flow is: >> >> hardware walks page table; L2 page table points to read only memory >> -> pf_interception (code = >> -> kvm_handle_page_fault (need_unprotect = false) >> -> kvm_mmu_page_fault >> -> paging64_page_fault (for example) >> -> try_async_pf >> map_writable set to false >> -> paging64_fetch(write_fault = true, map_writable = false, >> prefault = false) >> -> mmu_set_spte(speculative = false, host_writable = false, >> write_fault = true) >> -> set_spte >> mmu_need_write_protect returns true >> return true >> write_fault == true -> set emulate = true >> return true >> return true >> return true >> emulate >> >> Without this patch, emulation would have called >> >> ..._gva_to_gpa_nested >> -> translate_nested_gpa >> -> paging64_gva_to_gpa >> -> paging64_walk_addr >> -> paging64_walk_addr_generic >> set fault (nested_page_fault=true) >> >> and then: >> >> kvm_propagate_fault >> -> nested_svm_inject_npf_exit >> > > maybe then safer thing would be to qualify the new error_code check with > !mmu_is_nested(vcpu) or something like that. So that way it would run on > L1 guest, and not the L2 guest. I believe that would restrict it avoid > hitting this case. Are you okay with this change ? Or check "vcpu->arch.mmu.direct_map"? That would be true when not using shadow pages. > IIRC, the main place where this check was valuable was when L1 guest had > a fault (when coming out of the L2 guest) and emulation was not needed. How do I measure the effect? I tried counting the number of emulations, and any difference from the patch was lost in noise. Paolo