linux-kernel - Re: [RFC PATCH v3 4/7] bus/cdx: add cdx-MSI domain with gic-its domain as parent

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Date:   Wed, 12 Oct 2022 10:02:29 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     "Radovanovic, Aleksandar" <aleksandar.radovanovic@....com>
Cc:     "Gupta, Nipun" <Nipun.Gupta@....com>,
        Marc Zyngier <maz@...nel.org>,
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        "git (AMD-Xilinx)" <git@....com>
Subject: Re: [RFC PATCH v3 4/7] bus/cdx: add cdx-MSI domain with gic-its
 domain as parent

On Wed, Oct 12, 2022 at 10:34:23AM +0000, Radovanovic, Aleksandar wrote:

> For the MSI EventID, the HW interrupt logic assumes the MSI write value is 
> equal to the MSI vector number. However, the vector number is programmable
> for most (all) of the interrupt sources, which isn't exactly the same as saying
> EventID is programmable for a vector number, but can be used to emulate the
> desired behaviour, with a translation table in firmware. 

If you do this stuff wrong you will eventually run into situations
that don't work. Like VFIO/VMs and things.

> As for GITS_TRANSLATER, we can take up to 4 different IOVAs, which limits us
> to 4 CDX devices (should be sufficient for current HW use-cases). Also, it means
> that the address part must be the same for all vectors within a single CDX 
> device. I'm assuming this is OK as it is going to be a single interrupt and IOMMU
> domain anyway.

This is not at all how MSI is supposed to work.

Jason