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dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:34476 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kaN4Z-0006pK-Ff for qemu-devel@archiver.kernel.org; Wed, 04 Nov 2020 12:59:51 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:40504) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kaN3F-0005om-Og for qemu-devel@nongnu.org; Wed, 04 Nov 2020 12:58:29 -0500 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:37098) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1kaN3B-0003rF-TS for qemu-devel@nongnu.org; Wed, 04 Nov 2020 12:58:29 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1604512703; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=wVz5GsJ1sllaoRHffQLdaGqfihET3aY01lnZ56RP9w8=; b=Whsp/3Y9aI1ITlTtFnufvrrGgQ3k1HObAKpnON3FEHmfmp85conrGp+ngE+pXriGlF2bCL 1QIXwG5/shcc0CJ2Wo7fGYpPCtilWbGuy38OmsCRgmG+Efw7N+Fw/FS/Ui6TGBtzNlH02Q Jm1Bg74esEy0majBK+KXuxe+mmt52Yw= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-521-gIvY-FNbPzOLSmvUD_vmVA-1; Wed, 04 Nov 2020 12:58:20 -0500 X-MC-Unique: gIvY-FNbPzOLSmvUD_vmVA-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id A2B7B10E2183 for ; Wed, 4 Nov 2020 17:58:19 +0000 (UTC) Received: from lacos-laptop-7.usersys.redhat.com (ovpn-112-163.ams2.redhat.com [10.36.112.163]) by smtp.corp.redhat.com (Postfix) with ESMTP id A38A75B4CC; Wed, 4 Nov 2020 17:58:09 +0000 (UTC) Subject: Re: [PATCH RFC] memory: pause all vCPUs for the duration of memory transactions To: Vitaly Kuznetsov , Peter Xu References: <20201026084916.3103221-1-vkuznets@redhat.com> <20201102195729.GA20600@xz-x1> <87v9emy4g2.fsf@vitty.brq.redhat.com> From: Laszlo Ersek Message-ID: <2e6e47d4-2a77-b8fb-723c-f38ec944057c@redhat.com> Date: Wed, 4 Nov 2020 18:58:08 +0100 MIME-Version: 1.0 In-Reply-To: <87v9emy4g2.fsf@vitty.brq.redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=lersek@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=lersek@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/11/03 00:03:41 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_H4=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 , 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" On 11/03/20 14:07, Vitaly Kuznetsov wrote: > 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. > Indeed that's how I seem to remember it too; the guest page tables cannot be walked (by the processor implicitly, or by KVM explicitly -- I can't tell which one of those applies). Thanks Laszlo