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 Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id E290CC433EF for ; Wed, 2 Feb 2022 07:28:10 +0000 (UTC) Received: from localhost ([::1]:54948 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1nFA3l-0003B4-90 for qemu-devel@archiver.kernel.org; Wed, 02 Feb 2022 02:28:09 -0500 Received: from eggs.gnu.org ([209.51.188.92]:58990) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nF9im-0007Q1-MZ for qemu-devel@nongnu.org; Wed, 02 Feb 2022 02:06:34 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]:39138) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nF9ii-00072Y-Qd for qemu-devel@nongnu.org; Wed, 02 Feb 2022 02:06:26 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1643785581; 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: in-reply-to:in-reply-to:references:references; bh=CyFcgqiAbqR7SXHOMKvTa4RcV+Fwn7uGY7WA8M5JNSI=; b=P6sGx/kyGun3fw6bQNxXiaVx1jk3CI7pRT+4/12JPDSDociJejMZD2PogeHcxQ/Ee9lYhV wB3YcMjf2zdaNohZQayGEm1exxRFe7Z0STy/vpqqPQ9XrUk6Wsay6OC5zziCnTzRGpA8Bl Es8NzbRcRpET+hxxn5CTyD7+jXGaxFY= Received: from mail-ed1-f70.google.com (mail-ed1-f70.google.com [209.85.208.70]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-417-ShPU47OnP8KY96NkhmpiYg-1; Wed, 02 Feb 2022 02:06:18 -0500 X-MC-Unique: ShPU47OnP8KY96NkhmpiYg-1 Received: by mail-ed1-f70.google.com with SMTP id ed6-20020a056402294600b004090fd8a936so9930895edb.23 for ; Tue, 01 Feb 2022 23:06:18 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=CyFcgqiAbqR7SXHOMKvTa4RcV+Fwn7uGY7WA8M5JNSI=; b=HV96LRNBPmxZQ2TLYR/Hap/aXaYi905GAjxkiD3l24YcNKkcP53wktmcJuXztpKpK4 WM4Cg5ixeEKqlE6CVmfzFC9CBa/hdNs6aQRSTTFwQNtXDj4KKkeELnDQwRJAoQcFKAAp HG/MfM3hUFLW72/zLhbLLS0B5IGmigJm4O/LPQjzQISneWnVqOl7nDwEMcF7n7HjO6FC badcog+nD2vi4CJ8VnH+i9QHoKoVVo88aWCOKI1oNwm2voPcpkqbbuCLeJAM1AIc/8pr EVwny4evkbeZpnwQNHBFuSaGPRgcdGwt5XTQOoOqGsQ+WqGAc0Trp1L1YehPwuzIrioj +uzQ== X-Gm-Message-State: AOAM531+3Le+tmk5pptaWBqNRg/D88v/384I3X54qeZplu2OIxSMQRn3 8cUOSDxuMb5LiSinTf1qLZvHiuM31bPKsut76czySvsNGkmbKWCDl4BDdKBVkBi+U6Bren3aoy3 RqxICKROqZS3fGhE= X-Received: by 2002:a17:907:6e0d:: with SMTP id sd13mr18556508ejc.582.1643785577189; Tue, 01 Feb 2022 23:06:17 -0800 (PST) X-Google-Smtp-Source: ABdhPJwJeb9vKIACMN6xMe/z/tVXOLQXwcPbzTtykOcjJBVJsdue2pigI8n+9KcuMS+5LmfCWYIIxw== X-Received: by 2002:a17:907:6e0d:: with SMTP id sd13mr18556469ejc.582.1643785576853; Tue, 01 Feb 2022 23:06:16 -0800 (PST) Received: from redhat.com ([2.52.5.34]) by smtp.gmail.com with ESMTPSA id lt12sm15237350ejb.166.2022.02.01.23.06.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 01 Feb 2022 23:06:16 -0800 (PST) Date: Wed, 2 Feb 2022 02:06:12 -0500 From: "Michael S. Tsirkin" To: Halil Pasic Subject: Re: [PATCH v3 1/1] virtio: fix the condition for iommu_platform not supported Message-ID: <20220202020543-mutt-send-email-mst@kernel.org> References: <20220201133915.3764972-1-pasic@linux.ibm.com> <20220201193309.7da86258.pasic@linux.ibm.com> <365305e3-4224-965d-2cb6-496a95802f0e@gmail.com> <20220202021547.20dc65c9.pasic@linux.ibm.com> MIME-Version: 1.0 In-Reply-To: <20220202021547.20dc65c9.pasic@linux.ibm.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mst@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=170.10.129.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.086, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Cornelia Huck , Brijesh Singh , Jason Wang , Daniel Henrique Barboza , qemu-devel@nongnu.org, Jakob Naucke , qemu-stable@nongnu.org, Daniel Henrique Barboza Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Feb 02, 2022 at 02:15:47AM +0100, Halil Pasic wrote: > On Tue, 1 Feb 2022 16:31:22 -0300 > Daniel Henrique Barboza wrote: > > > On 2/1/22 15:33, Halil Pasic wrote: > > > On Tue, 1 Feb 2022 12:36:25 -0300 > > > Daniel Henrique Barboza wrote: > > > > > >>> + vdev_has_iommu = virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM); > > >>> if (klass->get_dma_as != NULL && has_iommu) { > > >>> virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM); > > >>> vdev->dma_as = klass->get_dma_as(qbus->parent); > > >>> + if (!vdev_has_iommu && vdev->dma_as != &address_space_memory) { > > >>> + error_setg(errp, > > >>> + "iommu_platform=true is not supported by the device"); > > >>> + } > > >> > > >> > > >>> } else { > > >>> vdev->dma_as = &address_space_memory; > > >>> } > > >> > > >> > > >> I struggled to understand what this 'else' clause was doing and I assumed that it was > > >> wrong. Searching through the ML I learned that this 'else' clause is intended to handle > > >> legacy virtio devices that doesn't support the DMA API (introduced in 8607f5c3072caeebb) > > >> and thus shouldn't set VIRTIO_F_IOMMU_PLATFORM. > > >> > > >> > > >> My suggestion, if a v4 is required for any other reason, is to add a small comment in this > > >> 'else' clause explaining that this is the legacy virtio devices condition and those devices > > >> don't set F_IOMMU_PLATFORM. This would make the code easier to read for a virtio casual like > > >> myself. > > > > > > I do not agree that this is about legacy virtio. In my understanding > > > virtio-ccw simply does not need translation because CCW devices use > > > guest physical addresses as per architecture. It may be considered > > > legacy stuff form PCI perspective, but I don't think it is legacy > > > in general. > > > > > > I wasn't talking about virtio-ccw. I was talking about this piece of code: > > > > > > if (klass->get_dma_as != NULL && has_iommu) { > > virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM); > > vdev->dma_as = klass->get_dma_as(qbus->parent); > > } else { > > vdev->dma_as = &address_space_memory; > > } > > > > > > I suggested something like this: > > > > > > > > if (klass->get_dma_as != NULL && has_iommu) { > > virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM); > > vdev->dma_as = klass->get_dma_as(qbus->parent); > > } else { > > /* > > * We don't force VIRTIO_F_IOMMU_PLATFORM for legacy devices, i.e. > > * devices that don't implement klass->get_dma_as, regardless of > > * 'has_iommu' setting. > > */ > > vdev->dma_as = &address_space_memory; > > } > > > > > > At least from my reading of commits 8607f5c3072 and 2943b53f682 this seems to be > > the case. I spent some time thinking that this IF/ELSE was wrong because I wasn't > > aware of this history. > > With virtio-ccw we take the else branch because we don't implement > ->get_dma_as(). I don't consider all the virtio-ccw to be legacy. > > IMHO there are two ways to think about this: > a) The commit that introduced this needs a fix which implemets > get_dma_as() for virtio-ccw in a way that it simply returns > address_space_memory. > b) The presence of ->get_dma_as() is not indicative of "legacy". > > BTW in virtospeak "legacy" has a special meaning: pre-1.0 virtio. Do you > mean that legacy. And if I read the virtio-pci code correctly > ->get_dma_as is set for legacy, transitional and modern devices alike. > > IMHO the important thing to figure out is what impact that > virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM); > in the first branch (of the if-else) has. IMHO if one examines the > commits 8607f5c307 ("virtio: convert to use DMA api") and 2943b53f68 > ("virtio: force VIRTIO_F_IOMMU_PLATFORM") very carefully, one will > probably reach the conclusion that the objective of the latter, is > to prevent the guest form not negotiating the IOMMU_PLATFORM feature > (clearing it as part of the feature negotiation) and trying to use > the device without that feature. In other words, virtio features are > usually optional for the guest for the sake of compatibility, but > IOMMU_PLATFORM is not: for very good reasons. Neither the commit message > nor the patch does mention legacy anywhere. > > In my opinion not forcing the guest to negotiate IOMMU_PLATFORM when > ->get_dma_as() is not set is at least unfortunate. Please observe, that > virtio-pci is not affected by this omission because for virtio-pci > devices ->get_dma_as != NULL always holds. And what is the deal for > devices that don't implement get_dma_as() (and don't need address > translation)? If iommu_platform=on is justified (no user error) then > the device does not have access to the entire guest memory. Which > means it more than likely needs cooperation form the guest (driver). > So detecting that the guest does not support IOMMU_PLATFORM and failing > gracefully via virtio_validate_features() instead of carrying on > in good faith and failing in ugly ways when the host attempts to access > guest memory to which it does not have access to. If we assume user > error, that is the host can access at least all the memory it needs > to access to make that device work, then it is probably still a > good idea to fail the device and thus help the user correct his > error. > > IMHO the best course of action is > diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c > index 34f5a0a664..1d0eb16d1c 100644 > --- a/hw/virtio/virtio-bus.c > +++ b/hw/virtio/virtio-bus.c > @@ -80,7 +80,6 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp) > > vdev_has_iommu = virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM); > if (klass->get_dma_as != NULL && has_iommu) { > - virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM); > vdev->dma_as = klass->get_dma_as(qbus->parent); > if (!vdev_has_iommu && vdev->dma_as != &address_space_memory) { > error_setg(errp, > @@ -89,6 +88,7 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp) > } else { > vdev->dma_as = &address_space_memory; > } > + virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM); > } > > which would be a separate patch, as this is a separate issue. Jason, > Michael, Connie, what do you think? Do you mean just force VIRTIO_F_IOMMU_PLATFORM for everyone? Or am I misreading the patch? > Regards, > Halil