linux-kernel - Re: [PATCH v7 13/13] Documentation: userspace-api: iommufd: Update vIOMMU

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Message-ID: <ZzVB7Fun48rGztuV@Asurada-Nvidia>
Date: Wed, 13 Nov 2024 16:18:52 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: Yi Liu <yi.l.liu@...el.com>
CC: <jgg@...dia.com>, <kevin.tian@...el.com>, <corbet@....net>,
	<joro@...tes.org>, <suravee.suthikulpanit@....com>, <will@...nel.org>,
	<robin.murphy@....com>, <dwmw2@...radead.org>, <shuah@...nel.org>,
	<iommu@...ts.linux.dev>, <linux-doc@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
	<baolu.lu@...ux.intel.com>, <eric.auger@...hat.com>,
	<jean-philippe@...aro.org>, <mdf@...nel.org>, <mshavit@...gle.com>,
	<shameerali.kolothum.thodi@...wei.com>, <smostafa@...gle.com>, <aik@....com>,
	<zhangfei.gao@...aro.org>, <patches@...ts.linux.dev>
Subject: Re: [PATCH v7 13/13] Documentation: userspace-api: iommufd: Update
 vIOMMU

On Tue, Nov 12, 2024 at 09:15:02PM +0800, Yi Liu wrote:
> On 2024/11/6 04:04, Nicolin Chen wrote:
> > +5. IOMMUFD_OBJ_VIOMMU can be only manually created via the IOMMU_VIOMMU_ALLOC
> > +   uAPI, provided a dev_id (for the device's physical IOMMU to back the vIOMMU)
> > +   and an hwpt_id (to associate the vIOMMU to a nesting parent HWPT_PAGING). The
> > +   iommufd core will link the vIOMMU object to the struct iommu_device that the
> > +   struct device is behind.
> 
> It looks to be reasonable to share the viommu_obj between devices behind
> the same physical IOMMU. This design seems no enforcement for it. So it's
> all up to userspace from what I got. :)

It enforces at the vDEVICE allocation:
	if (viommu->iommu_dev != __iommu_get_iommu_dev(idev->dev)) {
		return -EINVAL;

Yet, assuming you are referring to creating two vIOMMUs per VM for
two devices behind the same IOMMU (?), there is no enforcement..

The suggested way for VMM, just like two devices sharing the same
s2 parent hwpt, is to share the vIOMMU object.

Thanks
Nic