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[34.83.136.168]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2b0bc76bc66sm10468195ad.13.2026.03.25.17.16.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 25 Mar 2026 17:16:29 -0700 (PDT) Date: Thu, 26 Mar 2026 00:16:25 +0000 From: Samiullah Khawaja To: Pranjal Shrivastava Cc: David Woodhouse , Lu Baolu , Joerg Roedel , Will Deacon , Jason Gunthorpe , Robin Murphy , Kevin Tian , Alex Williamson , Shuah Khan , iommu@lists.linux.dev, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Saeed Mahameed , Adithya Jayachandran , Parav Pandit , Leon Romanovsky , William Tu , Pratyush Yadav , Pasha Tatashin , David Matlack , Andrew Morton , Chris Li , Vipin Sharma , YiFei Zhu Subject: Re: [PATCH 12/14] iommufd: Add APIs to preserve/unpreserve a vfio cdev Message-ID: References: <20260203220948.2176157-1-skhawaja@google.com> <20260203220948.2176157-13-skhawaja@google.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: On Wed, Mar 25, 2026 at 09:23:13PM +0000, Pranjal Shrivastava wrote: >On Wed, Mar 25, 2026 at 08:41:46PM +0000, Samiullah Khawaja wrote: >> On Wed, Mar 25, 2026 at 08:24:24PM +0000, Pranjal Shrivastava wrote: >> > On Tue, Feb 03, 2026 at 10:09:46PM +0000, Samiullah Khawaja wrote: >> > > Add APIs that can be used to preserve and unpreserve a vfio cdev. Use >> > > the APIs exported by the IOMMU core to preserve/unpreserve device. Pass >> > > the LUO preservation token of the attached iommufd into IOMMU preserve >> > > device API. This establishes the ownership of the device with the >> > > preserved iommufd. >> > > >> > > Signed-off-by: Samiullah Khawaja >> > > --- >> > > drivers/iommu/iommufd/device.c | 69 ++++++++++++++++++++++++++++++++++ >> > > include/linux/iommufd.h | 23 ++++++++++++ >> > > 2 files changed, 92 insertions(+) >> > > >> > > diff --git a/drivers/iommu/iommufd/device.c b/drivers/iommu/iommufd/device.c >> > > index 4c842368289f..30cb5218093b 100644 >> > > --- a/drivers/iommu/iommufd/device.c >> > > +++ b/drivers/iommu/iommufd/device.c >> > > @@ -2,6 +2,7 @@ >> > > /* Copyright (c) 2021-2022, NVIDIA CORPORATION & AFFILIATES >> > > */ >> > > #include >> > > +#include >> > > #include >> > > #include >> > > #include >> > > @@ -1661,3 +1662,71 @@ int iommufd_get_hw_info(struct iommufd_ucmd *ucmd) >> > > iommufd_put_object(ucmd->ictx, &idev->obj); >> > > return rc; >> > > } >> > > + >> > > +#ifdef CONFIG_IOMMU_LIVEUPDATE >> > > +int iommufd_device_preserve(struct liveupdate_session *s, >> > > + struct iommufd_device *idev, >> > > + u64 *tokenp) >> > > +{ >> > > + struct iommufd_group *igroup = idev->igroup; >> > > + struct iommufd_hwpt_paging *hwpt_paging; >> > > + struct iommufd_hw_pagetable *hwpt; >> > > + struct iommufd_attach *attach; >> > > + int ret; >> > > + >> > > + mutex_lock(&igroup->lock); >> > > + attach = xa_load(&igroup->pasid_attach, IOMMU_NO_PASID); >> > >> > By explicitly looking up IOMMU_NO_PASID, we skip any PASID attachments >> > the device might have. Since PASID live update is NOT supported in this >> > series, should we check if the pasid_attach xarray contains anything >> > other than IOMMU_NO_PASID and return -EOPNOTSUPP? >> > >> > Otherwise, we silently fail to preserve those domains without informing >> > the VMM? >> >> VMM should be able to preserve the NO_PASID domains even if it has PASID >> attachments. This is the intended behaviour, I will document it in the >> uAPI docs. > >I think I'm miscommunicating here. My concern isn't about whether the >kernel can mechanically preserve the NO_PASID domain when PASID >attachments exist. I agree that part works fine. > >My concern is purely about silent state loss. If a VMM asks the kernel >to preserve a device, it expects the entire IOMMU state for that device >to be safely handed over. If the kernel silently skips the PASID >attachments and returns success (0), the VMM on the new kernel will wake >up assuming those PASIDs are still perfectly intact. When the guest >attempts a PASID-tagged DMA, it will unexpectedly fault. This is a valid concern about silent state loss and I should have been elaborate in my reply earlier. My thinking is that PASID preservation should be a granular and opt-in feature. - PRESERVE_DEVICE only preserves the IOMMU_NO_PASID attachment. - A future patch would add mechanism to mark PASID attachment for preservation. The VMM would have to call this for each specific PASID it wants to preserve. > >So the question is: how strictly should the kernel protect userspace >from this footgun? A few options that I can see: > >1. Rely on uAPI docs >2. Fail the preserve ioctl (-EOPNOTSUPP) if active PASID attachments > are detected. >3. Add an opt-in flag: We could add a flag to the ioctl > (IOMMU_LU_FLAG_IGNORE_PASID) so userspace has to explicitly > acknowledge the state drop? > >Options 2 or 3 are especially important when we consider backwards >compatibility. If this series is merged in 7.2 with the "silent drop" >behavior now, when full PASID live update support is eventually added >in a future kernel, userspace will have no robust way to know if it's >running on a kernel that preserves PASIDs or silently drops them. By >returning an error or requiring a flag now, we reserve the right to >cleanly implement the feature later without breaking the UAPI contract. I think these are valid points and I will return -EOPNOTSUPP if active PASID attachments are detected. I will update this in next revision. > >This is an open question from me, I'm okay with any of the 3 options >I'd like to know what the maintainers think about this as well. > >[ ---- >8 ----- ] > >Thanks, >Praan Thanks, Sami