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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]:43668 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kXOkv-0002n0-Cl for qemu-devel@archiver.kernel.org; Tue, 27 Oct 2020 09:11:17 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:48938) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kXOj2-0001Kf-Br for qemu-devel@nongnu.org; Tue, 27 Oct 2020 09:09:20 -0400 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:42375) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1kXOiz-00045i-VR for qemu-devel@nongnu.org; Tue, 27 Oct 2020 09:09:19 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1603804157; 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=OUpRSTVxyxlttb9u/NAhRjLMMqc5boby3PLv/IPdtVQ=; b=gP2/ReJqo69CMkEsjBrX3c+t8q8aAn6RPm4q1JsxQpNBS1zsAPrWNZM796l0kImPsjIrn9 RILXndTssgIyL8J343NhL7h/0gYBaHsBRXNzwwlGfFHfPtxGoNxGmNtaYsFYT5Ajj9+6SK ymqcyGnSkYHgYgdMDrL+WlvBklcdEY4= 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-576-rm6SHvXDNNmC8EgdiAXilA-1; Tue, 27 Oct 2020 09:09:14 -0400 X-MC-Unique: rm6SHvXDNNmC8EgdiAXilA-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 2620F18FE883 for ; Tue, 27 Oct 2020 13:08:54 +0000 (UTC) Received: from [10.36.113.185] (ovpn-113-185.ams2.redhat.com [10.36.113.185]) by smtp.corp.redhat.com (Postfix) with ESMTP id 9506F10013C0; Tue, 27 Oct 2020 13:08:49 +0000 (UTC) Subject: Re: [PATCH RFC] memory: pause all vCPUs for the duration of memory transactions To: Vitaly Kuznetsov , qemu-devel@nongnu.org References: <20201026084916.3103221-1-vkuznets@redhat.com> <294a987d-b0ef-1b58-98ac-0d4d43075d6e@redhat.com> <87imav26d8.fsf@vitty.brq.redhat.com> <87a6w72565.fsf@vitty.brq.redhat.com> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: Date: Tue, 27 Oct 2020 14:08:48 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <87a6w72565.fsf@vitty.brq.redhat.com> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=david@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=63.128.21.124; envelope-from=david@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/10/27 01:06:07 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -42 X-Spam_score: -4.3 X-Spam_bar: ---- X-Spam_report: (-4.3 / 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, NICE_REPLY_A=-2.167, 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 , Eduardo Habkost , Peter Xu , "Dr. David Alan Gilbert" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 27.10.20 14:02, Vitaly Kuznetsov wrote: > David Hildenbrand writes: > >> On 27.10.20 13:36, Vitaly Kuznetsov wrote: >>> David Hildenbrand writes: >>> >>>> On 26.10.20 11:43, David Hildenbrand wrote: >>>>> On 26.10.20 09:49, 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. >>>>>> >>>>>> An ideal solution to the problem would probably require KVM to provide a >>>>>> new API to do the whole transaction in one shot but as a band-aid we can >>>>>> just pause all vCPUs to make memory transations atomic. >>>>>> >>>>>> Reported-by: Dr. David Alan Gilbert >>>>>> Signed-off-by: Vitaly Kuznetsov >>>>>> --- >>>>>> RFC: Generally, memap updates happen only a few times during guest boot but >>>>>> I'm not sure there are no scenarios when pausing all vCPUs is undesireable >>>>>> from performance point of view. Also, I'm not sure if kvm_enabled() check >>>>>> is needed. >>>>>> Signed-off-by: Vitaly Kuznetsov >>>>>> --- >>>>>> softmmu/memory.c | 11 +++++++++-- >>>>>> 1 file changed, 9 insertions(+), 2 deletions(-) >>>>>> >>>>>> diff --git a/softmmu/memory.c b/softmmu/memory.c >>>>>> index fa280a19f7f7..0bf6f3f6d5dc 100644 >>>>>> --- a/softmmu/memory.c >>>>>> +++ b/softmmu/memory.c >>>>>> @@ -28,6 +28,7 @@ >>>>>> >>>>>> #include "exec/memory-internal.h" >>>>>> #include "exec/ram_addr.h" >>>>>> +#include "sysemu/cpus.h" >>>>>> #include "sysemu/kvm.h" >>>>>> #include "sysemu/runstate.h" >>>>>> #include "sysemu/tcg.h" >>>>>> @@ -1057,7 +1058,9 @@ static void address_space_update_topology(AddressSpace *as) >>>>>> void memory_region_transaction_begin(void) >>>>>> { >>>>>> qemu_flush_coalesced_mmio_buffer(); >>>>>> - ++memory_region_transaction_depth; >>>>>> + if ((++memory_region_transaction_depth == 1) && kvm_enabled()) { >>>>>> + pause_all_vcpus(); >>>>>> + } >>>>>> } >>>>>> >>>>>> void memory_region_transaction_commit(void) >>>>>> @@ -1087,7 +1090,11 @@ void memory_region_transaction_commit(void) >>>>>> } >>>>>> ioeventfd_update_pending = false; >>>>>> } >>>>>> - } >>>>>> + >>>>>> + if (kvm_enabled()) { >>>>>> + resume_all_vcpus(); >>>>>> + } >>>>>> + } >>>>>> } >>>>>> >>>>>> static void memory_region_destructor_none(MemoryRegion *mr) >>>>>> >>>>> >>>>> This is in general unsafe. pause_all_vcpus() will temporarily drop the >>>>> BQL, resulting in bad things happening to caller sites. >>> >>> Oh, I see, thanks! I was expecting there's a reason we don't have this >>> simple fix in already :-) >>> >>>>> >>>>> I studies the involved issues quite intensively when wanting to resize >>>>> memory regions from virtio-mem code. It's not that easy. >>>>> >>>>> Have a look at my RFC for resizing. You can apply something similar to >>>>> other operations. >>>>> >>>>> https://www.mail-archive.com/qemu-devel@nongnu.org/msg684979.html >>>> >>>> Oh, and I even mentioned the case you try to fix here back then >>>> >>>> " >>>> Instead of inhibiting during the region_resize(), we could inhibit for the >>>> hole memory transaction (from begin() to commit()). This could be nice, >>>> because also splitting of memory regions would be atomic (I remember there >>>> was a BUG report regarding that), however, I am not sure if that might >>>> impact any RT users. >>>> " >>>> >>>> The current patches live in >>>> https://github.com/davidhildenbrand/qemu/commits/virtio-mem-next >>>> >>>> Especially >>>> >>>> https://github.com/davidhildenbrand/qemu/commit/433fbb3abed20f15030e42f2b2bea7e6b9a15180 >>>> >>>> >>> >>> I'm not sure why we're focusing on ioctls here. I was debugging my case >>> quite some time ago but from what I remember it had nothing to do with >>> ioctls from QEMU. When we are removing a memslot any exit to KVM may >>> trigger an error condition as we'll see that vCPU or some of our >>> internal structures (e.g. VMCS for a nested guest) references >>> non-existent memory. I don't see a good solution other than making the >>> update fully atomic from *all* vCPUs point of view and this requires >>> stopping all CPUs -- either from QEMU or from KVM. >> >> I cannot follow. My patch waits until *any* KVM ioctls are out of the >> kernel. That includes VCPUs, but also other ioctls (because there are >> some that require a consistent memory block state). >> >> So from a KVM point of view, the CPUs are stopped. > > Sorry for not being clear: your patch looks good to me, what I tried to > say is that with the current KVM API the only way to guarantee atomicity > of the update is to make vCPUs stop (one way or another), kicking them > out and preventing new IOCTLs from being dispatched is one way > (temporary pausing them inside KVM would be another, for example -- but > that would require *new* API supplying the whole transaction and not one > memslot update). Ah, got it. Yes - and I briefly looked into resizing slots inside KVM atomically and it already turned out to be a major pain. All that metadata that's allocated for a memory slot based on the size is problematic. Same applies to all other kinds of operations (splitting, punching out, ...) as you also mentioned. -- Thanks, David / dhildenb