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(e.d.c.5.4.7.0.e.f.a.2.f.e.2.0.1.4.d.3.1.6.b.8.0.0.b.8.0.1.0.0.2.ip6.arpa. [2001:8b0:8b6:13d4:102e:f2af:e074:5cde]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-488fb7bf7besm135201705e9.34.2026.04.21.10.46.39 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 21 Apr 2026 10:46:40 -0700 (PDT) Message-ID: Date: Tue, 21 Apr 2026 18:46:38 +0100 Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] vfio/pci: Set up VFIO barmap before creating a DMABUF Content-Language: en-GB From: Matt Evans To: "Tian, Kevin" , Alex Williamson Cc: Ankit Agrawal , Jason Gunthorpe , Yishai Hadas , Shameer Kolothum , Alistair Popple , Leon Romanovsky , "Kasireddy, Vivek" , Kees Cook , Zhi Wang , Peter Xu , Alexey Kardashevskiy , Eric Auger , "kvm@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "virtualization@lists.linux.dev" References: <20260415181423.1008458-1-mattev@meta.com> <20260416164406.7fb3bb0b@shazbot.org> <3fd01aa0-955d-44e3-aa4a-1918f9c42bb0@meta.com> In-Reply-To: <3fd01aa0-955d-44e3-aa4a-1918f9c42bb0@meta.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Proofpoint-ORIG-GUID: iVSfFo1UtsHgpgOW8pTf9xgfzX-qOWWD X-Authority-Analysis: v=2.4 cv=HLnz0Itv c=1 sm=1 tr=0 ts=69e7b802 cx=c_pps a=IwH782EDBk/vqbJ9rM8UFw==:117 a=xqWC_Br6kY4A:10 a=IkcTkHD0fZMA:10 a=A5OVakUREuEA:10 a=VkNPw1HP01LnGYTKEx00:22 a=7x6HtfJdh03M6CCDgxCd:22 a=03ozwUkBphtHgyqjj1sw:22 a=VwQbUJbxAAAA:8 a=VabnemYjAAAA:8 a=r1p2_3pzAAAA:8 a=ydsvmwYgiboQMSsmNQ4A:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 a=Ng9YvVcppn9CIdWre3nD:22 a=gKebqoRLp9LExxC7YDUY:22 a=r_pkcD-q9-ctt7trBg_g:22 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNDIxMDE3NiBTYWx0ZWRfX6VT6Ync9UEMH QWO3mMYrQAOoQ/IRQ8Wpzt8sLP1i9KVu47xxjyZ7uI05OY1JeLkZjge0nt6AjtYikfZi0oBEOd9 IUgBsYXb120N82NmBk13MNEzXx2MU60UMDgmobwItSqEUOkv4wQUwnBKnU2nWMLYNF5Jmq8zEuj 6rpsWcuxuxLbfVRpI0I+/2R6VU+7UcDtx89l8bzgIwPthLC2zokiZZXYIbYGW08mWJ8V0pAGMp5 bVJt8dmXCtgF1wmpmNfh/s5IOEaJAhmd4n8hNzLfj8YDuVwj2zx5P1b0idCdfeSj0T0I4V/lC1A muUSuhlHsfSA3nfw8e5QnDkVU4U4igwVdDs93klCCR7mdFQSv1hTr9BD5L8/adD4CZzyZ3qnvMV oNrho+19TTkWtk6z+glGJmBz5ai9YgO9hyK/mjbUFenb0YAXErhXfEazBvPe1uzgu+REDBWFBxz gCO/NN7O/2oa5yLMkUw== X-Proofpoint-GUID: iVSfFo1UtsHgpgOW8pTf9xgfzX-qOWWD X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.51,FMLib:17.12.100.49 definitions=2026-04-21_03,2026-04-21_02,2025-10-01_01 Hi, On 17/04/2026 20:11, Matt Evans wrote: > Hi Kevin, Alex, > > On 17/04/2026 06:16, Tian, Kevin wrote: >> >>> From: Alex Williamson >>> Sent: Friday, April 17, 2026 6:44 AM >>> >>> On Wed, 15 Apr 2026 11:14:22 -0700 >>> Matt Evans wrote: >>> >>>> A DMABUF exports access to BAR resources which need to be requested >>>> before the DMABUF is handed out.  Usually the resources are requested >>>> when setting up the barmap when the VFIO device fd is mmap()ed, but >>>> there's no guarantee that's done before a DMABUF is created. >>>> >>>> Set up the barmap (and so request resources) in the DMABUF-creation >>>> path. >>>> >>>> Fixes: 5d74781ebc86c ("vfio/pci: Add dma-buf export support for MMIO >>> regions") >>>> Signed-off-by: Matt Evans >>>> --- >>>>   drivers/vfio/pci/vfio_pci_dmabuf.c | 9 +++++++++ >>>>   1 file changed, 9 insertions(+) >>>> >>>> diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c >>> b/drivers/vfio/pci/vfio_pci_dmabuf.c >>>> index 4ccaf3531e02..fefe7cf4256b 100644 >>>> --- a/drivers/vfio/pci/vfio_pci_dmabuf.c >>>> +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c >>>> @@ -272,6 +272,15 @@ int vfio_pci_core_feature_dma_buf(struct >>> vfio_pci_core_device *vdev, u32 flags, >>>>           goto err_free_priv; >>>>       } >>>> >>>> +    /* >>>> +     * See comment in vfio_pci_core_mmap(); ensure PCI regions >>>> +     * were requested before returning DMABUFs that reference >>>> +     * them.  Barmap setup does this: >>>> +     */ >>>> +    ret = vfio_pci_core_setup_barmap(vdev, get_dma_buf.region_index); >>>> +    if (ret) >>>> +        goto err_free_phys; >>>> + >>>>       priv->vdev = vdev; >>>>       priv->nr_ranges = get_dma_buf.nr_ranges; >>>>       priv->size = length; >>> >>> Wouldn't this get a lot easier if we just setup all the barmaps in >>> vfio_pci_core_enable(), conditional on pci_resource_len() just like we >>> use to filter in REGION_INFO? >>> >>> I don't recall if there's some reason we've avoid this so far, maybe >>> others can shout it out if they do. >> >> I don't remember too. probably just because it's not a wide requirement >> then was made in this on-demand approach... >> >>> >>> We already tear them all down in vfio_pci_core_disable().  It would be >>> a small patch to add that, which we would mark as Fixes:, then a small >>> follow-up on top of that that removes any then redundant or unnecessary >>> callers (all of them).  Thoughts?  Thanks, >>> >> >> Agree. then the next patch fixing the racing conditions is also not >> required. > > Oh, if we can do it earlier it's all much cleaner, agreed.  I'd naively > thought there was a reason it was done lazily.  One benefit is truly > huge TB-scale BARs don't get ioremap()ed until actually used by > userspace.  I can't think of a good usage example that relies on open > performance. > > If you say fine, then great:  Ignore these, and I'll post a new pair of > patches doing that (...with a cover letter this time). New series posted: https://lore.kernel.org/kvm/20260421174143.3883579-1-mattev@meta.com/ (Titles/semantics very different, so didn't send as a v2 of this.) Matt