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Re: [PATCH v5 1/8] xen/arm: introduce static shared memory





On 29/06/2022 06:38, Penny Zheng wrote:
Hi Julien

Hi Penny,


-----Original Message-----
From: Julien Grall <julien@xxxxxxx>
Sent: Saturday, June 25, 2022 1:55 AM
To: Penny Zheng <Penny.Zheng@xxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
Cc: Wei Chen <Wei.Chen@xxxxxxx>; Stefano Stabellini
<sstabellini@xxxxxxxxxx>; Bertrand Marquis <Bertrand.Marquis@xxxxxxx>;
Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
Subject: Re: [PATCH v5 1/8] xen/arm: introduce static shared memory

Hi Penny,

On 20/06/2022 06:11, Penny Zheng wrote:
From: Penny Zheng <penny.zheng@xxxxxxx>

This patch serie introduces a new feature: setting up static

Typo: s/serie/series/

shared memory on a dom0less system, through device tree configuration.

This commit parses shared memory node at boot-time, and reserve it in
bootinfo.reserved_mem to avoid other use.

This commits proposes a new Kconfig CONFIG_STATIC_SHM to wrap
static-shm-related codes, and this option depends on static memory(
CONFIG_STATIC_MEMORY). That's because that later we want to reuse a
few helpers, guarded with CONFIG_STATIC_MEMORY, like
acquire_staticmem_pages, etc, on static shared memory.

Signed-off-by: Penny Zheng <penny.zheng@xxxxxxx>
Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
---
v5 change:
- no change
---
v4 change:
- nit fix on doc
---
v3 change:
- make nr_shm_domain unsigned int
---
v2 change:
- document refinement
- remove bitmap and use the iteration to check
- add a new field nr_shm_domain to keep the number of shared domain
---
   docs/misc/arm/device-tree/booting.txt | 120
++++++++++++++++++++++++++
   xen/arch/arm/Kconfig                  |   6 ++
   xen/arch/arm/bootfdt.c                |  68 +++++++++++++++
   xen/arch/arm/include/asm/setup.h      |   3 +
   4 files changed, 197 insertions(+)

diff --git a/docs/misc/arm/device-tree/booting.txt
b/docs/misc/arm/device-tree/booting.txt
index 98253414b8..6467bc5a28 100644
--- a/docs/misc/arm/device-tree/booting.txt
+++ b/docs/misc/arm/device-tree/booting.txt
@@ -378,3 +378,123 @@ device-tree:

   This will reserve a 512MB region starting at the host physical address
   0x30000000 to be exclusively used by DomU1.
+
+Static Shared Memory
+====================
+
+The static shared memory device tree nodes allow users to statically
+set up shared memory on dom0less system, enabling domains to do
+shm-based communication.
+
+- compatible
+
+    "xen,domain-shared-memory-v1"
+
+- xen,shm-id
+
+    An 8-bit integer that represents the unique identifier of the shared
memory
+    region. The maximum identifier shall be "xen,shm-id = <0xff>".
+
+- 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. The number of
cells
+    for the host address (and size) is the same as the guest pseudo-physical
+    address and they are inherited from the parent node.

Sorry for jump in the discussion late. But as this is going to be a stable ABI, 
I
would to make sure the interface is going to be easily extendable.

AFAIU, with your proposal the host physical address is mandatory. I would
expect that some user may want to share memory but don't care about the
exact location in memory. So I think it would be good to make it optional in
the binding.

I think this wants to be done now because it would be difficult to change the
binding afterwards (the host physical address is the first set of cells).

The Xen doesn't need to handle the optional case.


Sure, I'll make "the host physical address" optional here, and right now, with 
no actual
code implementation. I'll make up it later in free time~

The user case you mentioned here is that we let xen to allocate an arbitrary 
static shared
memory region, so size and guest physical address are still mandatory, right?

That's correct.

Cheers,

--
Julien Grall



 


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