Hi,
Replying to Penny and Stefano answer in the same e-mail.
On 08/09/2022 04:21, Penny Zheng wrote:
-----Original Message-----
From: Stefano Stabellini <[email protected]>
Sent: Thursday, September 8, 2022 8:16 AM
To: Julien Grall <[email protected]>
Cc: Penny Zheng <[email protected]>; [email protected];
Wei Chen <[email protected]>; Stefano Stabellini
<[email protected]>; Bertrand Marquis <[email protected]>;
Volodymyr Babchuk <[email protected]>
Subject: Re: [PATCH v7 7/9] xen/arm: create shared memory nodes in guest
device tree
On Wed, 7 Sep 2022, Julien Grall wrote:
On 06/09/2022 09:59, Penny Zheng wrote:
We expose the shared memory to the domU using the "xen,shared-
memory-v1"
reserved-memory binding. See
Documentation/devicetree/bindings/reserved-memory/xen,shared-
memory.
txt in Linux for the corresponding device tree binding.
To save the cost of re-parsing shared memory device tree
configuration when creating shared memory nodes in guest device
tree, this commit adds new field "shm_mem" to store shm-info per
domain.
For each shared memory region, a range is exposed under the
/reserved-memory node as a child node. Each range sub-node is named
xen-shmem@<address> and has the following properties:
- compatible:
compatible = "xen,shared-memory-v1"
- reg:
the base guest physical address and size of the shared
memory region
- xen,id:
a string that identifies the shared memory region.
So technically, there is a property "xen,offset" that should be
specified for the borrowers.
TBH, I don't quite understand what this property is used for. So it is
not quite clear why this is skipped.
The Stefano is the author of the binding. So maybe he can explain the
purpose of the property and help to document it in the commit message
why this is ignored.
It looks like it is something we introduced to handle the case where memory
from the owner is shared with multiple borrowers. Each borrower would
have its own offset within the region shared by the owner:
https://marc.info/?l=xen-devel&m=154110446604365&w=2
Thanks for the pointer :). Now, I remember what this was meant for.
IMHO, "xen,offset" more fits in the xen dts? We configure it in borrower memory
node,
then later we shall only set up foreign memory map starting at the offset?
Yes and Yes.
'''
domU1-shared-mem@10000000 {
compatible = "xen,domain-shared-memory-v1";
role = "borrower";
xen,shm-id = "my-shared-mem-0";
xen,shared-mem = <0x10000000 0x50000000 0x10000000>;
xen,offset = <0x0 0x01000000>;
}
'''
For borrower domain, only [0x11000000, 0x20000000) need to get mapped...
Of course, we could limit the memory map in related Linux driver, but for
safety,
it should be at Xen?
Correct.
The use-case is a bit of a corner case but it looks valid. If I had to do it
now, I
would at least mark "xen,offset" as "optional".
I think we have two options here and I am happy with either one:
1) we add xen,offset = <0x0>;
I will let v8 stay with this configuration, and a TODO for actual
implementation.
Agree. It sounds the best approach for now.
Cheers,
--
Julien Grall
