Re: [PATCH] swiotlb: Allow swiotlb to live at pre-defined address

[Alexander Graf]

On 26.03.20 18:05, Christoph Hellwig wrote:
> On Thu, Mar 26, 2020 at 05:29:22PM +0100, Alexander Graf wrote:
> > The swiotlb is a very convenient fallback mechanism for bounce buffering of
> > DMAable data. It is usually used for the compatibility case where devices
> > can only DMA to a "low region".
> >
> > However, in some scenarios this "low region" may be bound even more
> > heavily. For example, there are embedded system where only an SRAM region
> > is shared between device and CPU. There are also heterogeneous computing
> > scenarios where only a subset of RAM is cache coherent between the
> > components of the system. There are partitioning hypervisors, where
> > a "control VM" that implements device emulation has limited view into a
> > partition's memory for DMA capabilities due to safety concerns.
> >
> > This patch adds a command line driven mechanism to move all DMA memory into
> > a predefined shared memory region which may or may not be part of the
> > physical address layout of the Operating System.
> >
> > Ideally, the typical path to set this configuration would be through Device
> > Tree or ACPI, but neither of the two mechanisms is standardized yet. Also,
> > in the x86 MicroVM use case, we have neither ACPI nor Device Tree, but
> > instead configure the system purely through kernel command line options.
> >
> > I'm sure other people will find the functionality useful going forward
> > though and extend it to be triggered by DT/ACPI in the future.
>
> I'm totally against hacking in a kernel parameter for this.  We'll need
> a proper documented DT or ACPI way.  

I'm with you on that sentiment, but in the environment I'm currently 
looking at, we have neither DT nor ACPI: The kernel gets purely 
configured via kernel command line. For other unenumerable artifacts on 
the system, such as virtio-mmio platform devices, that works well enough 
and also basically "hacks a kernel parameter" to specify the system layout.

> We also need to feed this information
> into the actual DMA bounce buffering decisions and not just the swiotlb
> placement.

Care to elaborate a bit here? I was under the impression that 
"swiotlb=force" basically allows you to steer the DMA bounce buffering 
decisions already.


Thanks!

Alex



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