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

[Oleksii Moisieiev <Oleksii_Moisieiev@xxxxxxxx>]

Hi Julien,

On Mon, Jan 03, 2022 at 01:14:17PM +0000, Julien Grall wrote:
> 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.

Do you mean that I can't use alloc_magic_pages to allocate shared
memory region for SCMI?

> 
> 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.

Could you please explain what do you mean under the "new region in the
memory map"?

> 
> 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.
> 

I'm using vmap call to map channel memory (see smc_create_channel()).
vmap call sets PAGE_HYPERVISOR flag which sets MT_NORMAL (0x7) flag.
Considering that protocol is synchronous and only one agent per channel is
expected - this works fine for now.
But I agree that the same memory attributes should be used in xen and
kernel. I fill fix that in v2.

Best regards,
Oleksii.