[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [RFC v1 3/5] xen/arm: introduce SCMI-SMC mediator driver



Hi,

On 24/12/2021 17:02, Oleksii Moisieiev wrote:
On Fri, Dec 24, 2021 at 03:42:42PM +0100, Julien Grall wrote:
On 20/12/2021 16:41, Oleksii Moisieiev wrote:
    2) What are the expected memory attribute for the regions?

xen uses iommu_permit_access to pass agent page to the guest. So guest can 
access the page directly.

I think you misunderstood my comment. Memory can be mapped with various type
(e.g. Device, Memory) and attribute (cacheable, non-cacheable...). What will
the firmware expect? What will the guest OS usually?

The reason I am asking is the attributes have to matched to avoid any
coherency issues. At the moment, if you use XEN_DOMCTL_memory_mapping, Xen
will configure the stage-2 to use Device nGnRnE. As the result, the result
memory access will be Device nGnRnE which is very strict.


Let me share with you the configuration example:
scmi expects memory to be configured in the device-tree:

cpu_scp_shm: scp-shmem@0xXXXXXXX {
        compatible = "arm,scmi-shmem";
        reg = <0x0 0xXXXXXX 0x0 0x1000>;
};

where XXXXXX address I allocate in alloc_magic_pages function.

The goal of alloc_magic_pages() is to allocate RAM. However, what you want is a guest physical address and then map a part of the shared page.

I can see two options here:
  1) Hardcode the SCMI region in the memory map
2) Create a new region in the memory map that can be used for reserving memory for mapping.

We still have plenty of space in the guest memory map. So the former is probably going to be fine for now.

Then I get paddr of the scmi channel for this domain and use
XEN_DOMCTL_memory_mapping to map scmi channel address to gfn.
 > Hope I wass able to answer your question.

What you provided me is how the guest OS will locate the shared memory. This still doesn't tell me which memory attribute will be used to map the page in Stage-1 (guest page-tables).

To find that out, you want to look at the driver and how the mapping is done. The Linux driver (drivers/firmware/arm_scmi) is using devm_ioremap() (see smc_chan_setup()).

Under the hood, the function devm_ioremap() is using PROT_DEVICE_nGnRE (arm64) which is one of the most restrictive memory attribute.

This means the firmware should be able to deal with the most restrictive attribute and therefore using XEN_DOMCTL_memory_mapping to map the shared page in stage-2 should be fine.

Cheers,

--
Julien Grall



 


Rackspace

Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.