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Date: Tue, 28 May 2024 13:33:23 -0700
From: Nicolin Chen <nicolinc@nvidia.com>
To: "Tian, Kevin" <kevin.tian@intel.com>
CC: Jason Gunthorpe <jgg@nvidia.com>, "will@kernel.org" <will@kernel.org>,
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Subject: Re: [PATCH RFCv1 08/14] iommufd: Add IOMMU_VIOMMU_SET_DEV_ID ioctl
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On Tue, May 28, 2024 at 01:22:46PM -0700, Nicolin Chen wrote:
> On Mon, May 27, 2024 at 01:08:43AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@nvidia.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?
> 
> What I mean is for regular vSMMU. Without VCMDQ, a regular vSMMU
> on a multi-pSMMU setup will look like (e.g. three devices behind
> different SMMUs):
> |<------ VMM ------->|<------ kernel ------>|
>        |-- viommu0 --|-- pSMMU0 --|
> vSMMU--|-- viommu1 --|-- pSMMU1 --|--s2_hwpt
>        |-- viommu2 --|-- pSMMU2 --|
> 
> And device would attach to:
> |<---- guest ---->|<--- VMM --->|<- kernel ->|
>        |-- dev0 --|-- viommu0 --|-- pSMMU0 --|
> vSMMU--|-- dev1 --|-- viommu1 --|-- pSMMU1 --|
>        |-- dev2 --|-- viommu2 --|-- pSMMU2 --|

I accidentally sent a duplicated one.. Please ignore this reply
and check the other one. Thanks!