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: <919bfaae-0a1d-de88-8211-f9fa476e6ebf@arm.com>
Date:   Thu, 8 Sep 2022 12:49:05 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     "Radovanovic, Aleksandar" <aleksandar.radovanovic@....com>,
        Marc Zyngier <maz@...nel.org>
Cc:     Jason Gunthorpe <jgg@...dia.com>,
        "Gupta, Nipun" <Nipun.Gupta@....com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "krzysztof.kozlowski+dt@...aro.org" 
        <krzysztof.kozlowski+dt@...aro.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "rafael@...nel.org" <rafael@...nel.org>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "cohuck@...hat.com" <cohuck@...hat.com>,
        "Gupta, Puneet (DCG-ENG)" <puneet.gupta@....com>,
        "song.bao.hua@...ilicon.com" <song.bao.hua@...ilicon.com>,
        "mchehab+huawei@...nel.org" <mchehab+huawei@...nel.org>,
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        "Simek, Michal" <michal.simek@....com>,
        "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 2022-09-08 10:51, Radovanovic, Aleksandar wrote:
> [AMD Official Use Only - General]
> 
> 
> 
>> -----Original Message-----
>> From: Marc Zyngier <maz@...nel.org>
>> Sent: 08 September 2022 09:08
>> To: Radovanovic, Aleksandar <aleksandar.radovanovic@....com>
>> Cc: Jason Gunthorpe <jgg@...dia.com>; Gupta, Nipun
>> <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; Gupta, Puneet (DCG-ENG)
>> <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;
>> robin.murphy@....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; Anand, Harpreet <harpreet.anand@....com>; Agarwal,
>> Nikhil <nikhil.agarwal@....com>; Simek, Michal <michal.simek@....com>;
>> git (AMD-Xilinx) <git@....com>
>> Subject: Re: [RFC PATCH v3 4/7] bus/cdx: add cdx-MSI domain with gic-its
>> domain as parent
>>
>> [CAUTION: External Email]
>>   
>> OK, so you definitely need a mapping, but it cannot be a translation, and it
>> needs to be in all the possible address spaces. OMG.
> 
> Could you elaborate why it needs to be in all the possible address spaces? I'm in no way familiar with kernel IOVA allocation, so not sure I understand this requirement. Note that each CDX device will have its own unique StreamID (in general case, equal to DeviceID sent to the GIC), so, from a SMMU perspective, the mapping can be specific to that device. As long as that IOVA is not allocated to any DMA region for _that_ device, things should be OK? But, I appreciate it might not be that simple from a kernel perspective.

That's the point - any device could could have its own mapping, 
therefore that hole has to be punched in *every* mapping that any of 
those devices could use, so that MSI writes don't unexpectedly fault, or 
corrupt memory if that address is free to be used to map a DMA buffer. 
At least the HiSilicon PCI quirk is functionally similar (for slightly 
different underlying reasons) so there's already precedent and an 
example that you can follow to a reasonable degree.

Robin.