linux-kernel - Re: [PATCH v3 24/30] vfio-pci/zdev: wire up group notifier

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Message-ID: <20220208204041.GK4160@nvidia.com>
Date:   Tue, 8 Feb 2022 16:40:41 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Matthew Rosato <mjrosato@...ux.ibm.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        linux-s390@...r.kernel.org, cohuck@...hat.com,
        schnelle@...ux.ibm.com, farman@...ux.ibm.com, pmorel@...ux.ibm.com,
        borntraeger@...ux.ibm.com, hca@...ux.ibm.com, gor@...ux.ibm.com,
        gerald.schaefer@...ux.ibm.com, agordeev@...ux.ibm.com,
        frankja@...ux.ibm.com, david@...hat.com, imbrenda@...ux.ibm.com,
        vneethv@...ux.ibm.com, oberpar@...ux.ibm.com, freude@...ux.ibm.com,
        thuth@...hat.com, pasic@...ux.ibm.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 24/30] vfio-pci/zdev: wire up group notifier

On Tue, Feb 08, 2022 at 03:33:58PM -0500, Matthew Rosato wrote:

> > Is the purpose of IOAT to associate the device to a set of KVM page
> > tables?  That seems like a container or future iommufd operation.  I
> 
> Yes, here we are establishing a relationship with the DMA table in the guest
> so that once mappings are established guest PCI operations (handled via
> special instructions in s390) don't need to go through the host but can be
> directly handled by firmware (so, effectively guest can keep running on its
> vcpu vs breaking out).

Oh, well, certainly sounds like a NAK on that - anything to do with
the DMA translation of a PCI device must go through the iommu layer,
not here.

Lets not repeat the iommu subsytem bypass mess power made please.

> It's more that non-KVM userspace doesn't care about what these ioctls are
> doing...  The enabling of 'interp, aif, ioat' is only pertinent when there
> is a KVM userspace, specifically because the information being shared /
> actions being performed as a result are only relevant to properly enabling
> zPCI features when the zPCI device is being passed through to a VM
> guest.

Then why are they KVM ioctls?

Jason