On 26 Jan 2022, at 10:58, Julien Grall <jul...@xen.org> wrote:
Hi,
On 26/01/2022 10:09, Penny Zheng wrote:
This commit provides a design doc for static shared memory
on a dom0less system.
Signed-off-by: Penny Zheng <penny.zh...@arm.com>
---
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+# Static Shared Memory between domains on a dom0less system
+
+This design aims to provide an overview of the new feature: setting up static
+shared memory between domains on a dom0less system, through device tree
+configuration.
+
+The new feature is driven by the need of finding a way to build up
+communication channels on dom0less system, since the legacy ways including
+grant table, etc are all absent there.
Stefano has a series to add support for grant-table [2]. So I think you want to
justify it differently.
+
+It was inspired by the patch serie of "xl/libxl-based shared memory", see
+[1] for more details.
+
+# Static Shared Memory Device Tree Configuration
+
+The static shared memory device tree nodes allow users to statically set up
+shared memory among a group of dom0less DomUs and Dom0, enabling domains
+to do shm-based communication.
+
+- compatible
+
+ "xen,domain-shared-memory-v1"
+
+- xen,shm-id
From the document, it is not clear to me what is the purpose of the
identifier. Could you clarify it?
+
+ An u32 value represents the unique identifier of the shared memory region.
+ User valuing per shared memory region shall follow the ascending order,
+ starting from xen,shm-id = <0x0>, to the maximum identifier
+ xen,shm-id = <0x126>.
Why is it limit to 0x126? And also, why do they have to be allocated in
ascending order?
The special xen,shm-id = <0x127> is reserved for
+ INVALID_SHMID.
Why do we need to reserve invalid?
+
+- xen,shared-mem
+
+ An array takes a physical address, which is the base address of the
+ shared memory region in host physical address space, a size, and a guest
+ physical address, as the target address of the mapping.
I think shared memory is useful without static allocation. So I think we want
to make the host physical address optional.
+
+- role(Optional)
+
+ A string property specifying the ownership of a shared memory region,
+ the value must be one of the following: "owner", or "borrower"
+ A shared memory region could be explicitly backed by one domain, which is
+ called "owner domain", and all the other domains who are also sharing
+ this region are called "borrower domain".
+ If not specified, the default value is "borrower" and owner is
+ "dom_shared", a system domain.
I don't particularly like adding another system domain. Instead, it would be
better to always specify the owner.
