linux-kernel - Re: [PATCH v1 01/14] iommu: Add iommu_get_unmanaged

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Message-ID: <ZBs0ifAVwWvOt2b5@nvidia.com>
Date:   Wed, 22 Mar 2023 14:02:01 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Eric Auger <eric.auger@...hat.com>
Cc:     Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>,
        Nicolin Chen <nicolinc@...dia.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "will@...nel.org" <will@...nel.org>,
        "kevin.tian@...el.com" <kevin.tian@...el.com>,
        "baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 01/14] iommu: Add iommu_get_unmanaged_domain helper

On Wed, Mar 22, 2023 at 05:07:39PM +0100, Eric Auger wrote:

> > It seems like Eric's issue is overly broad if we just want to block
> > RID reassignment that doesn't impact MMIO layout.
> IORT spec says
> 
> "
> If reserved memory regions are present, the OS must preserve PCIe
> configuration performed by the boot
> firmware. This preservation is required to ensure functional continuity
> of the endpoints that are using the reserved
> memory regions. Therefore, RMR nodes must be supported by the inclusion
> of the PCI Firmware defined _DSM
> for ignoring PCI boot configuration, Function 5, in the ACPI device
> object of the PCIe host bridge in ACPI
> namespace. The _DSM method should return a value of 0 to indicate that
> the OS must honour the PCI
> configuration that the firmware has done at boot time. See [PCIFW] for
> more details on this _DSM method.
> "

I would say this spec language is overly broad. If the FW knows the
reserved memory regions it creates are not sensitive to PCI layout
then it should not be forced to set this flag.

> > But, still, why do we care about this?
> >
> > The vIOMMU should virtualize the vSIDs right? So why does qemu give a
> > vSID list to the guest anyhow? Shouldn't the guest use an algorithmic
> > calculation from the vRID so that qemu can reverse it to the correct
> > vPCI device and thus the correct vfio_device and then dev id in the
> > iommu_domain?
> I don't understand how this changes the above picture?

We are forced to use RMR because of the hacky GIC ITS stuff.

ITS placement is not sensitive to PCI layout.

ITS is not sensitive to bus numbers/etc.

vSID to dev_id should also be taken care of by QEMU even if bus
numbers change and doesn't need to be fixed.

So let's have a reason why we need to do all this weird stuff beyond
the spec says so.

If there is no actual functional issue we should not restrict the
guest and provide RMR without the DSM method. Someone should go and
update the spec if this offends them :)

Jason