linux-kernel - RE: [PATCH RFCv1 08/14] iommufd: Add IOMMU_VIOMMU_SET_DEV

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Message-ID: <BN9PR11MB5276BBD592021507C3A0EBB38CF02@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Mon, 27 May 2024 01:08:43 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: Nicolin Chen <nicolinc@...dia.com>, "will@...nel.org" <will@...nel.org>,
	"robin.murphy@....com" <robin.murphy@....com>,
	"suravee.suthikulpanit@....com" <suravee.suthikulpanit@....com>,
	"joro@...tes.org" <joro@...tes.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "iommu@...ts.linux.dev"
	<iommu@...ts.linux.dev>, "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-tegra@...r.kernel.org"
	<linux-tegra@...r.kernel.org>, "Liu, Yi L" <yi.l.liu@...el.com>,
	"eric.auger@...hat.com" <eric.auger@...hat.com>, "vasant.hegde@....com"
	<vasant.hegde@....com>, "jon.grimm@....com" <jon.grimm@....com>,
	"santosh.shukla@....com" <santosh.shukla@....com>, "Dhaval.Giani@....com"
	<Dhaval.Giani@....com>, "shameerali.kolothum.thodi@...wei.com"
	<shameerali.kolothum.thodi@...wei.com>
Subject: RE: [PATCH RFCv1 08/14] iommufd: Add IOMMU_VIOMMU_SET_DEV_ID ioctl

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Friday, May 24, 2024 9:19 PM
> 
> On Fri, May 24, 2024 at 07:13:23AM +0000, Tian, Kevin wrote:
> > I'm curious to learn the real reason of that design. Is it because you
> > want to do certain load-balance between viommu's or due to other
> > reasons in the kernel smmuv3 driver which e.g. cannot support a
> > viommu spanning multiple pSMMU?
> 
> Yeah, there is no concept of support for a SMMUv3 instance where it's
> command Q's can only work on a subset of devices.
> 
> My expectation was that VIOMMU would be 1:1 with physical iommu
> instances, I think AMD needs this too??
> 

Yes this part is clear now regarding to VCMDQ.

But Nicoline said:

"
One step back, even without VCMDQ feature, a multi-pSMMU setup
will have multiple viommus (with our latest design) being added
to a viommu list of a single vSMMU's. Yet, vSMMU in this case
always traps regular SMMU CMDQ, so it can do viommu selection
or even broadcast (if it has to).
"

I don't think there is an arch limitation mandating that?