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: <Y0gH8R8tEqn6sqZ5@ziepe.ca>
Date:   Thu, 13 Oct 2022 09:43:29 -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 03:09:26PM +0000, Radovanovic, Aleksandar wrote:

> > 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.
> > 
> 
> I'm not sure I follow your argument, the limitation here is that the MSI
> address value is shared between vectors of the same device (requester id
> or endpoint, whichever way you prefer to call it), not between
> devices.

That isn't what you said, you said "we can take up to 4 different
IOVAs, which limits us to 4 CDX devices" - which sounds like HW being
shared across devices??

Jason