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: <Y0lq8vB2PT0zKUAQ@ziepe.ca>
Date:   Fri, 14 Oct 2022 10:58:10 -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 Fri, Oct 14, 2022 at 11:18:36AM +0000, Radovanovic, Aleksandar wrote:

> And that still does not imply lack of ordering or sharing of MSI
> target addresses between devices.

Either the end point generates the MSI, and maybe the bridge mangles
it, or it raises a lot of suspicion that this is not right. If the end
point generates the MSI then it raises the question why do we need to
tolerate these limits?

> This is a highly programmable IP block, at the core of which is an
> interconnect interfacing to programmable logic (PL), a number of
> PCIe controllers (either endpoint or root-port), DMA engines,
> offload engines, the embedded processor subsystem (PSX), etc. DMA
> and interrupts can be routed across it in almost any (meaningful)
> direction. The datapath 'endpoints' request DMA and interrupts, but
> don't concern themselves with the mechanics of delivering that in
> the target domain. It is the responsibility of the egress bridges to
> the target domains to convert the interconnect interrupt
> transactions to whatever the interrupt delivery mechanism for that
> domain is. E.g. for PCIe controllers in endpoint mode, that would be
> through PCIe MSI-X tables internal to the controller (and managed by
> the PCIe host), for PSX that would be the PSX bridge (partially
> managed by the PSX OS, mediated through firmware, i.e. through CDX
> bus driver) and so on. It is the responsibility of the interconnect
> to maintain transaction ordering (including DMA vs. interrupts). It
> is the responsibility of the firmware to manage the bridges
> according to the implemented use-case, so everything works as
> expected.

Again, this all just seems wrongly designed. MSI should not be part
of an interconnect bridge. We did that already 20 years ago, it was
called IOAPICs on x86 and I think everyone is happy to see it gone.

If you want to build IOAPICs again, I guess you can, but that is a
slightly different SW setup than the MSI you are trying to use here,
and even that didn't have the same limitations you are proposing.

> So, yes, the hardware that translates interrupt transactions to GIC
> AXI writes is shared between endpoints, but what I said above still
> applies. And that doesn't necessarily make it weird/wrong, it's just
> more complex than you might think.

If it doesn't fit the architecture, then I think it must be considered
wrong. Mis-using platform architected components like MSI in HW is
problematic.

You should design the HW properly so you don't have these
problems. Involving FW in the MSI setup is also a bad idea, POWER did
this and it made a big mess of their arch code :(

Jason