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From:   Thomas Gleixner <tglx@linutronix.de>
To:     "Raj\, Ashok" <ashok.raj@intel.com>
Cc:     Jason Gunthorpe <jgg@nvidia.com>,
        Dave Jiang <dave.jiang@intel.com>, vkoul@kernel.org,
        megha.dey@intel.com, maz@kernel.org, bhelgaas@google.com,
        alex.williamson@redhat.com, jacob.jun.pan@intel.com,
        yi.l.liu@intel.com, baolu.lu@intel.com, kevin.tian@intel.com,
        sanjay.k.kumar@intel.com, tony.luck@intel.com, jing.lin@intel.com,
        dan.j.williams@intel.com, kwankhede@nvidia.com,
        eric.auger@redhat.com, parav@mellanox.com, rafael@kernel.org,
        netanelg@mellanox.com, shahafs@mellanox.com,
        yan.y.zhao@linux.intel.com, pbonzini@redhat.com,
        samuel.ortiz@intel.com, mona.hossain@intel.com,
        dmaengine@vger.kernel.org, linux-kernel@vger.kernel.org,
        x86@kernel.org, linux-pci@vger.kernel.org, kvm@vger.kernel.org,
        Ashok Raj <ashok.raj@intel.com>
Subject: Re: [PATCH v3 05/18] dmaengine: idxd: add IMS support in base driver
In-Reply-To: <20201001010706.GD26492@otc-nc-03>
References: <160021207013.67751.8220471499908137671.stgit@djiang5-desk3.ch.intel.com> <160021248979.67751.3799965857372703876.stgit@djiang5-desk3.ch.intel.com> <87sgazgl0b.fsf@nanos.tec.linutronix.de> <20200930185103.GT816047@nvidia.com> <20200930214941.GB26492@otc-nc-03> <87d023gc71.fsf@nanos.tec.linutronix.de> <20201001010706.GD26492@otc-nc-03>
Date:   Thu, 01 Oct 2020 10:44:58 +0200
Message-ID: <87y2kqfi7p.fsf@nanos.tec.linutronix.de>
MIME-Version: 1.0
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On Wed, Sep 30 2020 at 18:07, Ashok Raj wrote:
> On Wed, Sep 30, 2020 at 11:57:22PM +0200, Thomas Gleixner wrote:
>
> Devices exposed to guest need host OS support for programming interrupt
> entries in the IOMMU interrupt remapping table. VFIO provides those 
> services for standard interrupt schemes like MSI/MSIx for instance. 
> Since IMS is device specific VFIO can't provide an intercept when 
> IMS entries are programmed by the guest OS.

Why is IMS exposed to the guest in the first place? You expose a
subdevice to a guest, right? And that subdevice should not even know
that IMS exists simply because IMS is strictly host specific.

The obvious emulation here is to make the subdevice look like a PCI
device and expose emulated MSIX (not MSI) which is intercepted when
accessing the MSIX table and then redirected to the proper place along
with IRTE and PASID and whatever.

>> Also this stuff is host side and not guest side. I seriously doubt that
>> you want to hand in the whole PCI device which contains the IMS
>
> You are right, but nothing prevents a user from simply taking a full PCI
> device and assign to guest. 

You surely can and should prevent that because it makes no sense and
cannot work.

That's why you want a generic check for 'this device is magic SIOV or
whatever' and not something burried deep into a driver..

Thanks,

        tglx