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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