linux-kernel - RE: [RFC] /dev/ioasid uAPI proposal

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Date:   Fri, 4 Jun 2021 07:33:22 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     Alex Williamson <alex.williamson@...hat.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Robin Murphy <robin.murphy@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        David Gibson <david@...son.dropbear.id.au>,
        Kirti Wankhede <kwankhede@...dia.com>,
        "David Woodhouse" <dwmw2@...radead.org>,
        Jason Wang <jasowang@...hat.com>
Subject: RE: [RFC] /dev/ioasid uAPI proposal

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Thursday, June 3, 2021 8:41 PM
> 
> > When discussing I/O page fault support in another thread, the consensus
> > is that an device handle will be registered (by user) or allocated (return
> > to user) in /dev/ioasid when binding the device to ioasid fd. From this
> > angle we can register {ioasid_fd, device_handle} to KVM and then call
> > something like ioasidfd_device_is_coherent() to get the property.
> > Anyway the coherency is a per-device property which is not changed
> > by how many I/O page tables are attached to it.
> 
> It is not device specific, it is driver specific
> 
> As I said before, the question is if the IOASID itself can enforce
> snoop, or not. AND if the device will issue no-snoop or not.

Sure. My earlier comment was based on the assumption that all IOASIDs
attached to a device should inherit the same snoop/no-snoop fact. But
looks it doesn't prevent a device driver from setting PTE_SNP only for
selected I/O page tables, according to whether isoch agents are involved.

An user space driver could figure out per-IOASID requirements itself.

A guest device driver can indirectly convey this information through 
vIOMMU.

Registering {IOASID_FD, IOASID} to KVM has another merit, as we also
need it to update CPU PASID mapping for ENQCMD. We can define
one interface for both requirements. 😊

Thanks
Kevin