linux-kernel - Re: [PATCH hyperv-next 5/6] arch, drivers: Add device struct bitfield to not bounce-buffer

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Message-ID: <20250410072354.GB32563@lst.de>
Date: Thu, 10 Apr 2025 09:23:54 +0200
From: Christoph Hellwig <hch@....de>
To: Dan Williams <dan.j.williams@...el.com>
Cc: Roman Kisel <romank@...ux.microsoft.com>,
	Robin Murphy <robin.murphy@....com>, aleksander.lobakin@...el.com,
	andriy.shevchenko@...ux.intel.com, arnd@...db.de, bp@...en8.de,
	catalin.marinas@....com, corbet@....net, dakr@...nel.org,
	dave.hansen@...ux.intel.com, decui@...rosoft.com,
	gregkh@...uxfoundation.org, haiyangz@...rosoft.com, hch@....de,
	hpa@...or.com, James.Bottomley@...senpartnership.com,
	Jonathan.Cameron@...wei.com, kys@...rosoft.com, leon@...nel.org,
	lukas@...ner.de, luto@...nel.org, m.szyprowski@...sung.com,
	martin.petersen@...cle.com, mingo@...hat.com, peterz@...radead.org,
	quic_zijuhu@...cinc.com, tglx@...utronix.de, wei.liu@...nel.org,
	will@...nel.org, iommu@...ts.linux.dev, linux-arch@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
	linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org, x86@...nel.org, apais@...rosoft.com,
	benhill@...rosoft.com, bperkins@...rosoft.com,
	sunilmut@...rosoft.com, Suzuki K Poulose <suzuki.poulose@....com>,
	linux-coco@...ts.linux.dev
Subject: Re: [PATCH hyperv-next 5/6] arch, drivers: Add device struct
 bitfield to not bounce-buffer

On Wed, Apr 09, 2025 at 04:30:17PM -0700, Dan Williams wrote:
> > Thanks, I should've highlighted that facet most certainly!
> 
> One would hope that no one is building a modern device with trusted I/O
> capability, *and* with a swiotlb addressing dependency. However, I agree
> that a non-shared swiotlb would be needed in such a scenario.

Hope is never a good idea when dealing with hardware :(  PCIe already
requires no addressing limitations, and programming interface specs
like NVMe double down on that.  But at least one big hyperscaler still
managed to build such a device.

Also even if the periphal device is not addressing limited, the root
port or interconnect might still be, we've seen quite a lot of that.