Re: [PATCH v2 8/8] xen/riscv: introduce early_fdt_map()

[Jan Beulich <jbeulich@suse.com>]

On 15.07.2024 15:58, Oleksii wrote:
> On Mon, 2024-07-15 at 10:52 +0200, Jan Beulich wrote:
>> On 12.07.2024 18:22, Oleksii Kurochko wrote:
>>> +    rc = map_pages_to_xen(BOOT_FDT_VIRT_START,
>>> maddr_to_mfn(base_paddr),
>>> +                          SZ_2M >> PAGE_SHIFT,
>>> +                          PAGE_HYPERVISOR_RO | _PAGE_BLOCK);
>>> +    if ( rc )
>>> +        panic("Unable to map the device-tree.\n");
>>> +
>>> +    offset = fdt_paddr % XEN_PT_LEVEL_SIZE(1);
>>> +    fdt_virt = (void *)BOOT_FDT_VIRT_START + offset;
>>> +
>>> +    if ( fdt_magic(fdt_virt) != FDT_MAGIC )
>>> +        return NULL;
>>> +
>>> +    size = fdt_totalsize(fdt_virt);
>>> +    if ( size > BOOT_FDT_VIRT_SIZE )
>>> +        return NULL;
>>> +
>>> +    if ( (offset + size) > SZ_2M )
>>> +    {
>>> +        rc = map_pages_to_xen(BOOT_FDT_VIRT_START + SZ_2M,
>>> +                              maddr_to_mfn(base_paddr + SZ_2M),
>>> +                              SZ_2M >> PAGE_SHIFT,
>>> +                              PAGE_HYPERVISOR_RO | _PAGE_BLOCK);
>>> +        if ( rc )
>>> +            panic("Unable to map the device-tree\n");
>>> +    }
>>
>> Why this two part mapping? And why are you mapping perhaps much more
>> than "size"?
> I wasn't able to find if RISC-V has a requirement for alignment of FDT
> address so I decided to follow Arm where FDT is required to be placed
> on a 8-byte boundary, so FDT can cross a 2MB boundary, so the second
> 2MB page should be mapped if the FDT is crossing the 2MB boundary.

This explains why you may need to map more than 2Mb (which wasn't the
question), but it doesn't explain why you need to do it in two steps.

Jan