linux-kernel - Re: [PATCH v3 3/4] PCI: Disable ACS SV capability for the broken IDT switches

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Message-ID: <20260206143014.GH943673@ziepe.ca>
Date: Fri, 6 Feb 2026 10:30:14 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Manivannan Sadhasivam <mani@...nel.org>
Cc: Bjorn Helgaas <helgaas@...nel.org>,
	Manivannan Sadhasivam <manivannan.sadhasivam@....qualcomm.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
	Naresh Kamboju <naresh.kamboju@...aro.org>,
	Pavankumar Kondeti <quic_pkondeti@...cinc.com>,
	Xingang Wang <wangxingang5@...wei.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Alex Williamson <alex@...zbot.org>,
	James Puthukattukaran <james.puthukattukaran@...cle.com>
Subject: Re: [PATCH v3 3/4] PCI: Disable ACS SV capability for the broken IDT
 switches

On Fri, Feb 06, 2026 at 02:41:36PM +0530, Manivannan Sadhasivam wrote:
> > It'd be worth expanding on this and what the effect of avoiding ACS SV
> > is.  Does this change which devices can be safely passed through to
> > virtual guests?  Does it give up isolation that users expect?
> > 
> 
> IMO, ACS SV is somewhat broken on this switch. But we can still passthrough the
> downstream devices to the guests. There won't be ACS SV apparently, but that's
> what users will get with broken hw.

I agree with this, the HW is very broken, let's have it at least work
properly in Linux on bare metal out of the box.

If someone really insists they need virtualization with narrow groups
on this HW then they need to come with a more complete fix. Using VFIO
is going to open up the reset flows that are problematic with the
current solution, so it isn't like that is already working fully.

Somehow I suspect nobody would use this switch for virtualization :)

Jason