From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A4FF9C43460 for ; Thu, 22 Apr 2021 00:02:11 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8C02461445 for ; Thu, 22 Apr 2021 00:02:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1343820AbhDVACo (ORCPT ); Wed, 21 Apr 2021 20:02:44 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:24909 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234886AbhDVACn (ORCPT ); Wed, 21 Apr 2021 20:02:43 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1619049729; h=from:from:reply-to: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=AWyEtL+6GA4nLFwMmyJC5Ypm9RaDP0KzLZdhIttrWhA=; b=LWi9i9xJwaJB4xtMCaXbgpVrkEH3OFEgJuztTmqSM86vNPxxHW0hWTsg3cH6RPtUMHruCD 6qJGffrqZU00Bvvy5stQGG1cxgsyjBoI6z5plzXwxmAfF4Q+ST6edZXme5J1rJZL93v4/f 3zjTTjfHH1pymWIF0dTIbdDU3F0huRg= 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-100-rpSKvb4KORWQllaxvzqZGA-1; Wed, 21 Apr 2021 20:01:56 -0400 X-MC-Unique: rpSKvb4KORWQllaxvzqZGA-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 E885B1006C82; Thu, 22 Apr 2021 00:01:54 +0000 (UTC) Received: from [10.64.54.94] (vpn2-54-94.bne.redhat.com [10.64.54.94]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1F8596064B; Thu, 22 Apr 2021 00:01:51 +0000 (UTC) Reply-To: Gavin Shan Subject: Re: [PATCH] KVM: arm64: Correctly handle the mmio faulting To: Marc Zyngier , Keqian Zhu Cc: kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org, cjia@nvidia.com, linux-arm-kernel@lists.infradead.org, "Wanghaibin (D)" References: <1603297010-18787-1-git-send-email-sashukla@nvidia.com> <8b20dfc0-3b5e-c658-c47d-ebc50d20568d@huawei.com> <2e23aaa7-0c8d-13ba-2eae-9e6ab2adc587@redhat.com> <871rb3rgpl.wl-maz@kernel.org> From: Gavin Shan Message-ID: Date: Thu, 22 Apr 2021 12:02:00 +1000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.0 MIME-Version: 1.0 In-Reply-To: <871rb3rgpl.wl-maz@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org Hi Marc, On 4/21/21 9:59 PM, Marc Zyngier wrote: > On Wed, 21 Apr 2021 07:17:44 +0100, > Keqian Zhu wrote: >> On 2021/4/21 14:20, Gavin Shan wrote: >>> On 4/21/21 12:59 PM, Keqian Zhu wrote: >>>> On 2020/10/22 0:16, Santosh Shukla wrote: >>>>> The Commit:6d674e28 introduces a notion to detect and handle the >>>>> device mapping. The commit checks for the VM_PFNMAP flag is set >>>>> in vma->flags and if set then marks force_pte to true such that >>>>> if force_pte is true then ignore the THP function check >>>>> (/transparent_hugepage_adjust()). >>>>> >>>>> There could be an issue with the VM_PFNMAP flag setting and checking. >>>>> For example consider a case where the mdev vendor driver register's >>>>> the vma_fault handler named vma_mmio_fault(), which maps the >>>>> host MMIO region in-turn calls remap_pfn_range() and maps >>>>> the MMIO's vma space. Where, remap_pfn_range implicitly sets >>>>> the VM_PFNMAP flag into vma->flags. >>>> Could you give the name of the mdev vendor driver that triggers this issue? >>>> I failed to find one according to your description. Thanks. >>>> >>> >>> I think it would be fixed in driver side to set VM_PFNMAP in >>> its mmap() callback (call_mmap()), like vfio PCI driver does. >>> It means it won't be delayed until page fault is issued and >>> remap_pfn_range() is called. It's determined from the beginning >>> that the vma associated the mdev vendor driver is serving as >>> PFN remapping purpose. So the vma should be populated completely, >>> including the VM_PFNMAP flag before it becomes visible to user >>> space. > > Why should that be a requirement? Lazy populating of the VMA should be > perfectly acceptable if the fault can only happen on the CPU side. > It isn't a requirement and the drivers needn't follow strictly. I checked several drivers before looking into the patch and found almost all the drivers have VM_PFNMAP set at mmap() time. In drivers/vfio/vfio-pci.c, there is a comment as below, but it doesn't reveal too much about why we can't set VM_PFNMAP at fault time. static int vfio_pci_mmap(void *device_data, struct vm_area_struct *vma) { : /* * See remap_pfn_range(), called from vfio_pci_fault() but we can't * change vm_flags within the fault handler. Set them now. */ vma->vm_flags |= VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP; vma->vm_ops = &vfio_pci_mmap_ops; return 0; } To set these flags in advance does have advantages. For example, VM_DONTEXPAND prevents the vma to be merged with another one. VM_DONTDUMP make this vma isn't eligible for coredump. Otherwise, the address space, which is associated with the vma is accessed and unnecessary page faults are triggered on coredump. VM_IO and VM_PFNMAP avoids to walk the page frames associated with the vma since we don't have valid PFN in the mapping. >>> >>> The example can be found from vfio driver in drivers/vfio/pci/vfio_pci.c: >>> vfio_pci_mmap: VM_PFNMAP is set for the vma >>> vfio_pci_mmap_fault: remap_pfn_range() is called >> Right. I have discussed the above with Marc. I want to find the driver >> to fix it. However, AFAICS, there is no driver matches the description... > > I have the feeling this is an out-of-tree driver (and Santosh email > address is bouncing, so I guess we won't have much information from > him). > > However, the simple fact that any odd driver can provide a fault > handler and populate it the VMA on demand makes me think that we need > to support this case. > Yeah, Santosh's email isn't reachable. I think the VM_PFNMAP need to be rechecked after gup if we're going to support this case. Thanks, Gavin