Re: [PATCH] xen/arm: Don't use copy_from_paddr for DTB relocation

["Orzel, Michal" <michal.orzel@xxxxxxx>]

On 26/02/2025 11:45, Luca Fancellu wrote:
> 
> 
> Hi Michal,
> 
>> On 26 Feb 2025, at 10:38, Orzel, Michal <michal.orzel@xxxxxxx> wrote:
>>
>>
>>
>> On 26/02/2025 09:36, Luca Fancellu wrote:
>>>
>>>
>>> Currently the early stage of the Arm boot maps the DTB using
>>> early_fdt_map() using PAGE_HYPERVISOR_RO which is cacheable
>>> read-only memory, later during DTB relocation the function
>>> copy_from_paddr() is used to map pages in the same range on
>>> the fixmap but using PAGE_HYPERVISOR_WC which is non-cacheable
>>> read-write memory.
>>>
>>> The Arm specifications, ARM DDI0487L.a, section B2.11 "Mismatched
>>> memory attributes" discourage using mismatched attributes for
>>> aliases of the same location.
>>>
>>> Given that there is nothing preventing the relocation since the region
>>> is already mapped, fix that by open-coding copy_from_paddr inside
>>> relocate_fdt, without mapping on the fixmap.
>>>
>>> Fixes: 1bdc81dac816 ("arm: setup MM using information from the device tree")
>> Why Fixes tag? I don't see it as a bug and something we need to backport...
> 
> ok I’ll remove it
> 
>>
>>> Signed-off-by: Luca Fancellu <luca.fancellu@xxxxxxx>
>>> ---
>>> xen/arch/arm/setup.c | 7 ++++---
>>> 1 file changed, 4 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
>>> index c1f2d1b89d43..b1fd4b44a2e1 100644
>>> --- a/xen/arch/arm/setup.c
>>> +++ b/xen/arch/arm/setup.c
>>> @@ -237,14 +237,15 @@ void __init discard_initial_modules(void)
>>> }
>>>
>>> /* Relocate the FDT in Xen heap */
>>> -static void * __init relocate_fdt(paddr_t dtb_paddr, size_t dtb_size)
>>> +static void * __init relocate_fdt(const void *dtb_vaddr, size_t dtb_size)
>>> {
>>>     void *fdt = xmalloc_bytes(dtb_size);
>>>
>>>     if ( !fdt )
>>>         panic("Unable to allocate memory for relocating the 
>>> Device-Tree.\n");
>>>
>>> -    copy_from_paddr(fdt, dtb_paddr, dtb_size);
>>> +    memcpy(fdt, dtb_vaddr, dtb_size);
>>> +    clean_dcache_va_range(dtb_vaddr, dtb_size);
>> The purpose of cleaning dcache after memcpy is to clean the new area i.e.
>> destination == fdt, not source region.
> 
> woops, my bad, I’ll fix
> 
>>
>>>
>>>     return fdt;
>>> }
>>> @@ -362,7 +363,7 @@ void asmlinkage __init start_xen(unsigned long 
>>> fdt_paddr)
>>>     if ( acpi_disabled )
>>>     {
>>>         printk("Booting using Device Tree\n");
>>> -        device_tree_flattened = relocate_fdt(fdt_paddr, fdt_size);
>>> +        device_tree_flattened = relocate_fdt(device_tree_flattened, 
>>> fdt_size);
>> NIT: It can be just my PoV but it reads odd. Why can't relocate_fdt modify
>> device_tree_flattened pointer directly in the function?
> 
> you mean something like:
> 
> static void * __init relocate_fdt(size_t dtb_size)
> {
> void *fdt = xmalloc_bytes(dtb_size);
> 
> if ( !fdt )
> panic("Unable to allocate memory for relocating the Device-Tree.\n");
> 
> memcpy(fdt, device_tree_flattened, dtb_size);
You already make assumption about device_tree_flattened being global, so why
not assigning device_tree_flattened = fdt in the function as well?

~Michal