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

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Date: Thu, 6 Jun 2024 21:27:07 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: "Tian, Kevin" <kevin.tian@...el.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

On Thu, Jun 06, 2024 at 11:44:58AM -0700, Nicolin Chen wrote:
> On Thu, Jun 06, 2024 at 03:24:23PM -0300, Jason Gunthorpe wrote:
> > On Sun, Jun 02, 2024 at 08:25:34PM -0700, Nicolin Chen wrote:
> > 
> > > > I understand the appeal of doing this has been to minimize qemu
> > > > changes in its ACPI parts if we tackle that instead maybe we should
> > > > just not implement viommu to multiple piommu. It is somewhat
> > > > complicated.
> > > 
> > > Would you please clarify that suggestion "not implement viommu
> > > to multiple piommu"?
> > > 
> > > For regular nesting (SMMU), we are still doing one vSMMU in the
> > > VMM, though VCMDQ case would be an exception....
> > 
> > This is what I mean, always do multiple vSMMU if there are multiple
> > physical pSMMUs. Don't replicate any virtual commands across pSMMUs.
> 
> Thanks for clarifying. That also means you'd prefer putting the
> command dispatcher in VMM, which is what we have at this moment.

Unless someone knows a reason why we should strive hard to have only a
single vSMMU and accept some invalidation inefficiency?

Jason