Re: [Xen-devel] [[PATCH for-4.13]] xen/arm: mm: Allow generic xen page-tables helpers to be called early

[Julien Grall <julien.grall@arm.com>]

Hi,

Gentle ping.

Cheers,

On 9/20/19 4:26 PM, Julien Grall wrote:
> Hi,
> 
> On 20/09/2019 16:16, Stefano Stabellini wrote:
>> On Fri, 20 Sep 2019, Julien Grall wrote:
>>> On 20/09/2019 00:37, Stefano Stabellini wrote:
>>>> On Tue, 17 Sep 2019, Julien Grall wrote:
>>>>> The current implementations of xen_{map, unmap}_table() expect
>>>>> {map, unmap}_domain_page() to be usable. Those helpers are used to
>>>>> map/unmap page tables while update Xen page-tables.
>>>>>
>>>>> Since commit 022387ee1a "xen/arm: mm: Don't open-code Xen PT update in
>>>>> {set, clear}_fixmap()", setup_fixmap() will make use of the helpers
>>>>> mentioned above. When booting Xen using GRUB, setup_fixmap() may be 
>>>>> used
>>>>> before map_domain_page() can be called. This will result to data 
>>>>> abort:
>>>>>
>>>>> (XEN) Data Abort Trap. Syndrome=0x5
>>>>> (XEN) CPU0: Unexpected Trap: Data Abort
>>>>>
>>>>> [...]
>>>>>
>>>>> (XEN) Xen call trace:
>>>>> (XEN)    [<000000000025ab6c>] mm.c#xen_pt_update+0x2b4/0x59c (PC)
>>>>> (XEN)    [<000000000025ab20>] mm.c#xen_pt_update+0x268/0x59c (LR)
>>>>> (XEN)    [<000000000025ae70>] set_fixmap+0x1c/0x2c
>>>>> (XEN)    [<00000000002a9c98>] copy_from_paddr+0x7c/0xdc
>>>>> (XEN)    [<00000000002a4ae0>] has_xsm_magic+0x18/0x34
>>>>> (XEN)    [<00000000002a5b5c>] bootfdt.c#early_scan_node+0x398/0x560
>>>>> (XEN)    [<00000000002a5de0>] device_tree_for_each_node+0xbc/0x144
>>>>> (XEN)    [<00000000002a5ed4>] boot_fdt_info+0x6c/0x260
>>>>> (XEN)    [<00000000002ac0d0>] start_xen+0x108/0xc74
>>>>> (XEN)    [<000000000020044c>] arm64/head.o#paging+0x60/0x88
>>>>>
>>>>> During early boot, the page tables are either statically allocated in
>>>>> Xen binary or allocated via alloc_boot_pages().
>>>>>
>>>>> For statically allocated page-tables, they will already be mapped as
>>>>> part of Xen binary. So we can easily find the virtual address.
>>>>>
>>>>> For dynamically allocated page-tables, we need to rely
>>>>> map_domain_page() to be functionally working.
>>>>>
>>>>> For arm32, the call will be usable much before page can be dynamically
>>>>> allocated (see setup_pagetables()). For arm64, the call will be usable
>>>>> after setup_xenheap_mappings().
>>>>>
>>>>> In both cases, memory are given to the boot allocator afterwards. 
>>>>> So we
>>>>> can rely on map_domain_page() for mapping page tables allocated
>>>>> dynamically.
>>>>>
>>>>> The helpers xen_{map, unmap}_table() are now updated to take into
>>>>> account the case where page-tables are part of Xen binary.
>>>>>
>>>>> Fixes: 022387ee1a ('xen/arm: mm: Don't open-code Xen PT update in 
>>>>> {set,
>>>>> clear}_fixmap()')
>>>>> Signed-off-by: Julien Grall <julien.grall@arm.com>
>>>>> ---
>>>>>    xen/arch/arm/mm.c | 20 ++++++++++++++++++++
>>>>>    1 file changed, 20 insertions(+)
>>>>>
>>>>> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
>>>>> index e1cdeaaf2f..da6303a8fd 100644
>>>>> --- a/xen/arch/arm/mm.c
>>>>> +++ b/xen/arch/arm/mm.c
>>>>> @@ -950,11 +950,31 @@ static int create_xen_table(lpae_t *entry)
>>>>>      static lpae_t *xen_map_table(mfn_t mfn)
>>>>>    {
>>>>> +    /*
>>>>> +     * We may require to map the page table before 
>>>>> map_domain_page() is
>>>>> +     * useable. The requirements here is it must be useable as 
>>>>> soon as
>>>>> +     * page-tables are allocated dynamically via alloc_boot_pages().
>>>>> +     */
>>>>> +    if ( system_state == SYS_STATE_early_boot )
>>>>> +    {
>>>>> +        vaddr_t va = mfn_to_maddr(mfn) - phys_offset;
>>>>> +
>>>>> +        if ( is_kernel(va) )
>>>>> +            return (lpae_t *)va;
>>>>
>>>> Is it intended to continue if it is not a xen text page? Shouldn't we
>>>> BUG() or WARN?
>>> Yes, I wrote the rationale in the commit message and a summary in a 
>>> few lines
>>> above. For convenience, I pasted the commit message again here:
>> The commit message explains what you are doing but I am still missing
>> something.
>>
>> Why are we continuing if system_state == SYS_STATE_early_boot and
>> !is_kernel(va)?
>>
>> The commit message explains that if system_state == SYS_STATE_early_boot
>> pagetable pages are static, right? 
> That's not correct. Below an excerpt of the commit message:
> 
> "During early boot, the page tables are either statically allocated in
> Xen binary or allocated via alloc_boot_pages()."
> 
> An example of dynamic allocation happening when system_state == 
> SYS_STATE_early_boot is in setup_xenheap_mappings(). alloc_boot_pages() 
> will be used to allocate intermediate page-tables as the runtime 
> allocator is not yet ready.
> 
>> Only after dynamic allocation are
>> possible it makes sense to use map_domain_page, and dynamic allocations
>> are possible roughly when system_state switched to SYS_STATE_boot.
> 
> That's not correct. alloc_boot_pages() is actually here to allow dynamic 
> allocation before the memory subsystem (and therefore the runtime 
> allocator) is initialized.
> 
> Half of the commit message actually explain when dynamic allocation can 
> be used. I am not entirely sure what is unclear in it so please suggest 
> a different commit message.
> 
> Cheers,
> 

-- 
Julien Grall

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