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: <4ca6383e-bd21-59bf-cc4e-cf3313164957@arm.com>
Date:   Wed, 7 Sep 2022 12:33:12 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Marc Zyngier <maz@...nel.org>, Jason Gunthorpe <jgg@...dia.com>
Cc:     Nipun Gupta <nipun.gupta@....com>, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, gregkh@...uxfoundation.org,
        rafael@...nel.org, eric.auger@...hat.com,
        alex.williamson@...hat.com, cohuck@...hat.com,
        puneet.gupta@....com, song.bao.hua@...ilicon.com,
        mchehab+huawei@...nel.org, f.fainelli@...il.com,
        jeffrey.l.hugo@...il.com, saravanak@...gle.com,
        Michael.Srba@...nam.cz, mani@...nel.org, yishaih@...dia.com,
        will@...nel.org, joro@...tes.org, masahiroy@...nel.org,
        ndesaulniers@...gle.com, linux-arm-kernel@...ts.infradead.org,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, kvm@...r.kernel.org, okaya@...nel.org,
        harpreet.anand@....com, nikhil.agarwal@....com,
        michal.simek@....com, aleksandar.radovanovic@....com, git@....com
Subject: Re: [RFC PATCH v3 4/7] bus/cdx: add cdx-MSI domain with gic-its
 domain as parent

On 2022-09-07 12:17, Marc Zyngier wrote:
> On Tue, 06 Sep 2022 18:19:06 +0100,
> Jason Gunthorpe <jgg@...dia.com> wrote:
>>
>> On Tue, Sep 06, 2022 at 07:17:58PM +0530, Nipun Gupta wrote:
>>
>>> +static void cdx_msi_write_msg(struct irq_data *irq_data,
>>> +			      struct msi_msg *msg)
>>> +{
>>> +	/*
>>> +	 * Do nothing as CDX devices have these pre-populated
>>> +	 * in the hardware itself.
>>> +	 */
>>> +}
>>
>> Huh?
>>
>> There is no way it can be pre-populated, the addr/data pair,
>> especially on ARM, is completely under SW control.
> 
> There is nothing in the GIC spec that says that.
> 
>> There is some commonly used IOVA base in Linux for the ITS page, but
>> no HW should hardwire that.
> 
> That's not strictly true. It really depends on how this block is
> integrated, and there is a number of existing blocks that know *in HW*
> how to signal an LPI.
> 
> See, as the canonical example, how the mbigen driver doesn't need to
> know about the address of GITS_TRANSLATER.
> 
> Yes, this messes with translation (the access is downstream of the
> SMMU) if you relied on it to have some isolation, and it has a "black
> hole" effect as nobody can have an IOVA that overlaps with the
> physical address of the GITS_TRANSLATER register.
> 
> But is it illegal as per the architecture? No. It's just stupid.

If that were the case, then we'd also need a platform quirk so the SMMU 
driver knows about it. Yuck.

But even then, are you suggesting there is some way to convince the ITS 
driver to allocate a specific predetermined EventID when a driver 
requests an MSI? Asking for a friend...

Cheers,
Robin.