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From: Pranjal Shrivastava <praan@google.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Dmitry Malkin <dma@nebius.com>, Will Deacon <will@kernel.org>,
	Robin Murphy <robin.murphy@arm.com>,
	Joerg Roedel <joro@8bytes.org>,
	Nicolin Chen <nicolinc@nvidia.com>,
	"iommu@lists.linux.dev" <iommu@lists.linux.dev>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"regressions@lists.linux.dev" <regressions@lists.linux.dev>,
	"stable@vger.kernel.org" <stable@vger.kernel.org>,
	NB-Core Team <NB-CoreTeam@nebius.com>
Subject: Re: [RFC PATCH] iommu/arm-smmu-v3: Allow nested attach for PCI bridges without vDEVICE
Date: Mon, 17 Aug 2026 12:53:36 +0000	[thread overview]
Message-ID: <aoMEUDQKlaWzvE8n@google.com> (raw)
In-Reply-To: <20260817114407.GB1175472@nvidia.com>

On Mon, Aug 17, 2026 at 08:44:07AM -0300, Jason Gunthorpe wrote:
> On Mon, Aug 17, 2026 at 11:16:17AM +0000, Pranjal Shrivastava wrote:
> > But, regarding PCI P2PDMA, you can still enable it with ACS enabled in
> > the upstream kernel *IF* your hardware supports TLP forwarding between
> > root ports.
> > 
> > When ACS is enabled, all TLPs get forwarded to the host's Root Ports
> > instead of getting routed directly through the switch.
> 
> Generally nobody wants to run like that, the performance is
> terrible. The reason these systems have a PCI switch is because of
> performance of the P2P path.

Ack. I was assuming the enablement was the major challenge. For perf we
definitly need to hairpin at the switch. 

> 
> Jason

Thanks,
Praan

  reply	other threads:[~2026-08-17 12:53 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2026-08-14 10:07 [RFC PATCH] iommu/arm-smmu-v3: Allow nested attach for PCI bridges without vDEVICE Dmitry Malkin
2026-08-14 14:15 ` Pranjal Shrivastava
2026-08-14 15:40   ` Dmitry Malkin
2026-08-14 17:49   ` Jason Gunthorpe
2026-08-14 17:48 ` Jason Gunthorpe
2026-08-14 18:56   ` Dmitry Malkin
2026-08-14 22:11     ` Jason Gunthorpe
2026-08-17  9:29       ` Dmitry Malkin
2026-08-17 11:16         ` Pranjal Shrivastava
2026-08-17 11:44           ` Jason Gunthorpe
2026-08-17 12:53             ` Pranjal Shrivastava [this message]
2026-08-17 11:41         ` Jason Gunthorpe
2026-08-17 14:26           ` Dmitry Malkin
2026-08-17 15:20             ` Jason Gunthorpe

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