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[91.219.240.2]) by smtp.gmail.com with ESMTPSA id u16sm10224672wrn.55.2020.11.03.05.07.09 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 03 Nov 2020 05:07:10 -0800 (PST) From: Vitaly Kuznetsov To: Peter Xu Subject: Re: [PATCH RFC] memory: pause all vCPUs for the duration of memory transactions In-Reply-To: <20201102195729.GA20600@xz-x1> References: <20201026084916.3103221-1-vkuznets@redhat.com> <20201102195729.GA20600@xz-x1> Date: Tue, 03 Nov 2020 14:07:09 +0100 Message-ID: <87v9emy4g2.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=vkuznets@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=63.128.21.124; envelope-from=vkuznets@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/11/03 01:02:05 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Paolo Bonzini , Laszlo Ersek , qemu-devel@nongnu.org, "Dr. David Alan Gilbert" , Eduardo Habkost Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Peter Xu writes: > Vitaly, > > On Mon, Oct 26, 2020 at 09:49:16AM +0100, Vitaly Kuznetsov wrote: >> Currently, KVM doesn't provide an API to make atomic updates to memmap when >> the change touches more than one memory slot, e.g. in case we'd like to >> punch a hole in an existing slot. >> >> Reports are that multi-CPU Q35 VMs booted with OVMF sometimes print something >> like >> >> !!!! X64 Exception Type - 0E(#PF - Page-Fault) CPU Apic ID - 00000003 !!!! >> ExceptionData - 0000000000000010 I:1 R:0 U:0 W:0 P:0 PK:0 SS:0 SGX:0 >> RIP - 000000007E35FAB6, CS - 0000000000000038, RFLAGS - 0000000000010006 >> RAX - 0000000000000000, RCX - 000000007E3598F2, RDX - 00000000078BFBFF >> ... >> >> The problem seems to be that TSEG manipulations on one vCPU are not atomic >> from other vCPUs views. In particular, here's the strace: >> >> Initial creation of the 'problematic' slot: >> >> 10085 ioctl(13, KVM_SET_USER_MEMORY_REGION, {slot=6, flags=0, guest_phys_addr=0x100000, >> memory_size=2146435072, userspace_addr=0x7fb89bf00000}) = 0 >> >> ... and then the update (caused by e.g. mch_update_smram()) later: >> >> 10090 ioctl(13, KVM_SET_USER_MEMORY_REGION, {slot=6, flags=0, guest_phys_addr=0x100000, >> memory_size=0, userspace_addr=0x7fb89bf00000}) = 0 >> 10090 ioctl(13, KVM_SET_USER_MEMORY_REGION, {slot=6, flags=0, guest_phys_addr=0x100000, >> memory_size=2129657856, userspace_addr=0x7fb89bf00000}) = 0 >> >> In case KVM has to handle any event on a different vCPU in between these >> two calls the #PF will get triggered. > > A pure question: Why a #PF? Is it injected into the guest? > Yes, we see a #PF injected in the guest during OVMF boot. > My understanding (which could be wrong) is that all thing should start with a > vcpu page fault onto the removed range, then when kvm finds that the memory > accessed is not within a valid memslot (since we're adding it back but not > yet), it'll become an user exit back to QEMU assuming it's an MMIO access. Or > am I wrong somewhere? In case it is a normal access from the guest, yes, but AFAIR here guest's CR3 is pointing to non existent memory and when KVM detects that it injects #PF by itself without a loop through userspace. -- Vitaly