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,

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:

"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."

Cheers,

-- 
Julien Grall

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