linux-kernel - Re: [PATCH v3 05/18] dmaengine: idxd: add IMS support in base driver

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Message-ID: <20200930223857.GV816047@nvidia.com>
Date:   Wed, 30 Sep 2020 19:38:57 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Raj, Ashok" <ashok.raj@...el.com>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Dave Jiang <dave.jiang@...el.com>, <vkoul@...nel.org>,
        <megha.dey@...el.com>, <maz@...nel.org>, <bhelgaas@...gle.com>,
        <alex.williamson@...hat.com>, <jacob.jun.pan@...el.com>,
        <yi.l.liu@...el.com>, <baolu.lu@...el.com>, <kevin.tian@...el.com>,
        <sanjay.k.kumar@...el.com>, <tony.luck@...el.com>,
        <jing.lin@...el.com>, <dan.j.williams@...el.com>,
        <kwankhede@...dia.com>, <eric.auger@...hat.com>,
        <parav@...lanox.com>, <rafael@...nel.org>, <netanelg@...lanox.com>,
        <shahafs@...lanox.com>, <yan.y.zhao@...ux.intel.com>,
        <pbonzini@...hat.com>, <samuel.ortiz@...el.com>,
        <mona.hossain@...el.com>, <dmaengine@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
        <linux-pci@...r.kernel.org>, <kvm@...r.kernel.org>
Subject: Re: [PATCH v3 05/18] dmaengine: idxd: add IMS support in base driver

On Wed, Sep 30, 2020 at 02:49:41PM -0700, Raj, Ashok wrote:

> One of the parameters it has is the "supported system page-sizes" which is
> usually there in the SRIOV properties. So it needed a place holder for
> that. 

No idea why this would be a PCI cap. It is certainly not something so
universal it needs standardizing. There are many ways a device can
manage a BAR to match a required protection granularity.

> When we provision an entire PCI device that is IMS capable. The guest
> driver does know it can update the IMS entries directly without going to
> the host. But in order to do remapping we need something like how we manage
> PASID allocation from guest, so an IRTE entry can be allocated and the host
> driver can write the proper values for IMS.

Look at the architecture we ended up with.

You need to make pci_subdevice_msi_create_irq_domain() fail if the
platform can't provide the functionality.

Working around that with PCI caps is pretty gross.

Jason