linux-kernel - Re: [PATCH V3] PCI: Extend ACS configurability

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Date: Mon, 10 Jun 2024 08:38:49 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: Vidya Sagar <vidyas@...dia.com>, "corbet@....net" <corbet@....net>,
	"bhelgaas@...gle.com" <bhelgaas@...gle.com>,
	Gal Shalom <galshalom@...dia.com>,
	Leon Romanovsky <leonro@...dia.com>,
	Thierry Reding <treding@...dia.com>,
	Jon Hunter <jonathanh@...dia.com>,
	Masoud Moshref Javadi <mmoshrefjava@...dia.com>,
	Shahaf Shuler <shahafs@...dia.com>,
	Vikram Sethi <vsethi@...dia.com>,
	Shanker Donthineni <sdonthineni@...dia.com>,
	Jiandi An <jan@...dia.com>, Tushar Dave <tdave@...dia.com>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Krishna Thota <kthota@...dia.com>,
	Manikanta Maddireddy <mmaddireddy@...dia.com>,
	"sagar.tv@...il.com" <sagar.tv@...il.com>,
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>
Subject: Re: [PATCH V3] PCI: Extend ACS configurability

On Fri, Jun 07, 2024 at 02:30:55PM -0500, Bjorn Helgaas wrote:
> "Correctly" is not quite the right word here; it's just a fact that
> the ACS settings determined at boot time result in certain IOMMU
> groups.  If the user desires different groups, it's not that something
> is "incorrect"; it's just that the user may have to accept less
> isolation to get the desired IOMMU groups.

That is not quite accurate.. There are HW configurations where ACS
needs to be a certain way for the HW to work with P2P at all. It isn't
just an optimization or the user accepts something, if they want P2P
at all they must get a ACS configuration appropriate for their system.

Jason