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

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Message-ID: <Y0bRZTP9Kc6mdCiu@ziepe.ca>
Date:   Wed, 12 Oct 2022 11:38:29 -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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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 01:37:54PM +0000, Radovanovic, Aleksandar wrote:
> > On Wed, Oct 12, 2022 at 10:34:23AM +0000, Radovanovic, Aleksandar wrote:
> > 
> > 
> > > 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.
> 
> In the general case, no, they're not.

I don't mean that you can hack this to work - I mean that in MSI the
addr/data is supposed to come from the end point itself, not from some
kind of shared structure. This is important because the actual act of
generating the write has to be coherent with the DMA the device is
doing, as the MSI write must push any DMA data to visibility to meet
the "producer / consumer" model.

So it is really weird/wrong to have a HW design where the MSI
infrastructure is shared across many devices.

Jason