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=-2.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 922A1C433B4 for ; Thu, 22 Apr 2021 08:01:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5C0D861426 for ; Thu, 22 Apr 2021 08:01:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229487AbhDVIB5 (ORCPT ); Thu, 22 Apr 2021 04:01:57 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34932 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230270AbhDVIBv (ORCPT ); Thu, 22 Apr 2021 04:01:51 -0400 Received: from mail-io1-xd32.google.com (mail-io1-xd32.google.com [IPv6:2607:f8b0:4864:20::d32]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DB6EDC06174A; Thu, 22 Apr 2021 01:00:24 -0700 (PDT) Received: by mail-io1-xd32.google.com with SMTP id g125so16005479iof.3; Thu, 22 Apr 2021 01:00:24 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=Wtfr7O2uONmx0ABungB4kEDY0XFNe5e+ojy+4iH8yEA=; b=p8PdxTO6IbO9u7UohErshlbh8icvmz/JDG7y1GqEgk5mFunkPsBVbEJUPRf78Xi2xr kSRUtcRVfjbmBxKG0qjsh1/HndZ29CxY5HD9LgYDZpvynQBepAi6pd6zM1ELQsCQQPg/ VWIbO9mbmcu338Af40wvjIcjKiGSYhoudTq00h3r+mwwNVgyw02UnLBE//yPuxQ6DeIU PvhCQFTnqLXragBst0XhiNBKvZYTZ++nQcoicTZ8MMJ4XXJFr6eXNsKDfAoxAQIk+qE2 S2fv6klbnyzH/XsIGBNYdcUU0vEp8S5guaaWAZS1jDJ+RyeiVvXtD9XfP1bDVA54/3k7 ke6Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=Wtfr7O2uONmx0ABungB4kEDY0XFNe5e+ojy+4iH8yEA=; b=YpKVfdRBtqEbu49p4i+3Fwakk7ax2C47jjtkIRzPCWP9WjuutsDr9WjG4RI/rzWwtG azCEhYCWYqlI9Pk0nS0f0A9iNh0weLzccqMeYEGruDUncEv2CP6zHWxdO84VEVVYZS6x fK8awCW76tZwl393HWroUB/cu/2HIwKrmgUhXG55YFYFsvHfcGrdrPVnl17UGAXSsUzt QNDzhGgyO+StJxCceEKiti6LpWZj55rbEC+3NEFttFf0acn1k8UydKcQGcjdz9m7x+5v bEnNrxhw1/su7KVSW1BeckIO+QrI73UyAOkf7TbAjomr/4jqflZoc9iNBOMKEVOqBam8 SJXg== X-Gm-Message-State: AOAM531g+wdAPS3QyrE/jlHdMCT3b5R/bV4SZozEvLfjn8F85RnTo5A2 pdSYebka9Xuy8uh4B7Oye8PYZ4ruDVXMXVk/9dM= X-Google-Smtp-Source: ABdhPJy1fKaPawu0LnQ0auRQy3Yko2GFa8uyOPy+TR/mQlHmW9sFgOxSaeUrz/hqzwMFlqoFPhsCfX6YuVrosEuiaXY= X-Received: by 2002:a05:6638:218b:: with SMTP id s11mr2060932jaj.81.1619078424192; Thu, 22 Apr 2021 01:00:24 -0700 (PDT) MIME-Version: 1.0 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> <87v98eq0dh.wl-maz@kernel.org> In-Reply-To: From: Santosh Shukla Date: Thu, 22 Apr 2021 13:30:13 +0530 Message-ID: Subject: Re: [PATCH] KVM: arm64: Correctly handle the mmio faulting To: "Tarun Gupta (SW-GPU)" Cc: Marc Zyngier , Gavin Shan , Keqian Zhu , 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)" Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 22, 2021 at 1:07 PM Tarun Gupta (SW-GPU) wrote: > > > > On 4/22/2021 12:20 PM, Marc Zyngier wrote: > > External email: Use caution opening links or attachments > > > > > > On Thu, 22 Apr 2021 03:02:00 +0100, > > Gavin Shan wrote: > >> > >> 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. > >>> Right. Hi keqian, You can refer to case http://lkml.iu.edu/hypermail/linux/kernel/2010.3/00952.html (Sorry Guys, I am not with nvidia, but My quick input.) > >> > >> 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. > > > > But PCI clearly isn't the case we are dealing with here, and not > > everything is VFIO either. I can *today* create a driver that > > implements a mmap+fault handler, call mmap() on it, pass the result to > > a memslot, and get to the exact same result Santosh describes. > > > > No PCI, no VFIO, just a random driver. We are *required* to handle > > that. > > Agree with Marc here, that kernel should be able to handle it without > VM_PFNMAP flag set in driver. > > For driver reference, you could check the V2 version of this patch that > got accepted upstream and has details as-to how this can be reproduced > using vfio-pci: https://www.spinics.net/lists/arm-kernel/msg848491.html > > > > > M. > > > > -- > > Without deviation from the norm, progress is not possible. > >