linux-kernel - Re: [PATCH v1 0/4] vfio/iommu: Flag to allow userspace to set DMA buffers system cacheable

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Message-ID: <20240826231749.GM3773488@nvidia.com>
Date: Mon, 26 Aug 2024 20:17:49 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Alex Williamson <alex.williamson@...hat.com>
Cc: Manoj Vishwanathan <manojvishy@...gle.com>,
	Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
	Joerg Roedel <joro@...tes.org>,
	linux-arm-kernel@...ts.infradead.org, kvm@...r.kernel.org,
	iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
	David Dillow <dillow@...gle.com>
Subject: Re: [PATCH v1 0/4] vfio/iommu: Flag to allow userspace to set DMA
 buffers system cacheable

On Mon, Aug 26, 2024 at 11:04:47AM -0600, Alex Williamson wrote:
> On Mon, 26 Aug 2024 07:16:37 +0000
> Manoj Vishwanathan <manojvishy@...gle.com> wrote:
> 
> > Hi maintainers,
> > 
> > This RFC patch introduces the ability for userspace to control whether
> > device (DMA) buffers are marked as cacheable, enabling them to utilize
> > the system-level cache.
> > 
> > The specific changes made in this patch are:
> > 
> > * Introduce a new flag in `include/linux/iommu.h`: 
> >     * `IOMMU_SYS_CACHE` -  Indicates if the associated page should be cached in the system's cache hierarchy.
> > * Add `VFIO_DMA_MAP_FLAG_SYS_CACHE` to `include/uapi/linux/vfio.h`:

You'll need a much better description of what this is supposed to do
when you resend it.

IOMMU_CACHE already largely means that pages should be cached.

So I don't know what ARM's "INC_OCACHE" actually is doing. Causing
writes to land in a cache somewhere in hierarchy? Something platform
specific? I have no idea. By your description it sounds similar to the
x86 data placement stuff, whatever that was called, and the more
modern TPH approach.

Jason