Re: [RFC 00/18] vfio: Adopt iommufd

From: Jason Gunthorpe <jgg@nvidia.com>
To: Alex Williamson <alex.williamson@redhat.com>
Cc: "Tian, Kevin" <kevin.tian@intel.com>,
	"Daniel P. Berrangé" <berrange@redhat.com>,
	"Liu, Yi L" <yi.l.liu@intel.com>,
	"akrowiak@linux.ibm.com" <akrowiak@linux.ibm.com>,
	"jjherne@linux.ibm.com" <jjherne@linux.ibm.com>,
	"Peng, Chao P" <chao.p.peng@intel.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"Laine Stump" <laine@redhat.com>,
	"libvir-list@redhat.com" <libvir-list@redhat.com>,
	"jasowang@redhat.com" <jasowang@redhat.com>,
	"cohuck@redhat.com" <cohuck@redhat.com>,
	"thuth@redhat.com" <thuth@redhat.com>,
	"peterx@redhat.com" <peterx@redhat.com>,
	"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>,
	"pasic@linux.ibm.com" <pasic@linux.ibm.com>,
	"eric.auger@redhat.com" <eric.auger@redhat.com>,
	"Sun, Yi Y" <yi.y.sun@intel.com>,
	"nicolinc@nvidia.com" <nicolinc@nvidia.com>,
	"eric.auger.pro@gmail.com" <eric.auger.pro@gmail.com>,
	"david@gibson.dropbear.id.au" <david@gibson.dropbear.id.au>
Subject: Re: [RFC 00/18] vfio: Adopt iommufd
Date: Tue, 26 Apr 2022 13:42:17 -0300	[thread overview]
Message-ID: <20220426164217.GR2125828@nvidia.com> (raw)
In-Reply-To: <20220426102159.5ece8c1f.alex.williamson@redhat.com>

On Tue, Apr 26, 2022 at 10:21:59AM -0600, Alex Williamson wrote:
> We also need to be able to advise libvirt as to how each iommufd object
> or user of that object factors into the VM locked memory requirement.
> When used by vfio-pci, we're only mapping VM RAM, so we'd ask libvirt
> to set the locked memory limit to the size of VM RAM per iommufd,
> regardless of the number of devices using a given iommufd.  However, I
> don't know if all users of iommufd will be exclusively mapping VM RAM.
> Combinations of devices where some map VM RAM and others map QEMU
> buffer space could still require some incremental increase per device
> (I'm not sure if vfio-nvme is such a device).  It seems like heuristics
> will still be involved even after iommufd solves the per-device
> vfio-pci locked memory limit issue.  Thanks,

If the model is to pass the FD, how about we put a limit on the FD
itself instead of abusing the locked memory limit?

We could have a no-way-out ioctl that directly limits the # of PFNs
covered by iopt_pages inside an iommufd.

Jason