Re: [Xen-devel] ACPI fixmap overflow

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.11.16 at 21:51, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 11/15/2016 03:44 PM, Andrew Cooper wrote:
>> On 15/11/2016 20:39, Boris Ostrovsky wrote:
>>> On 11/15/2016 02:45 PM, Andrew Cooper wrote:
>>>> On 15/11/16 19:34, Boris Ostrovsky wrote:
>>>>> In addition to running out of e820 entries on that large machine that
>>>>> Alex was referring to in [0] he is also running out of ACPI fixmap space
>>>>> while parsing MADT (the box has *lots* of processors). The
>>>>> quick-and-dirty solution is to increase NUM_FIXMAP_ACPI_PAGES but I
>>>>> wonder whether we should move to dynamic memory allocation.
>>>> Why do we use fixmap anyway?  It doesn't look too hard to reorder
>>>> vm_init() slightly higher, and be able to use ioremap() for all APCI 
>>>> tables.
>>> Hmm... Let me see how possible this is. Just moving it up won't work
>>> since heap allocator is initialized after ACPI tables.
>> We have plenty of usable PTEs already allocated at boot, mainly from the
>> init pagetables.  Given a static __init vm_bitmap, a new boot-time-only
>> vm range should be usable without any heap allocations at all.
> 
> Wouldn't that (using pre-allocated PTEs), in a way, be equivalent to
> increasing fixmap size?

Indeed. For the time being I think growing the fixmap should be
fine. Clearly it being a fixed 4 pages has been wrong for a long
time - there's no point in it being smaller than MAX_LOCAL_APIC
x2apic entries, plus min(MAX_LOCAL_APIC, 256) lapic ones, plus
MAX_IO_APICS ioapic ones etc.

Otoh the actual parsing of MADT happens after the heap allocator
has been initialized. The only earlier use is via acpi_table_init() ->
check_multiple_madt(); acpi_initialize_tables() doesn't appear to
map full tables. And check_multiple_madt() not being able to map
the full table would not prevent boot from continuing afaics. So
Andrew's suggestion to switch to dynamic mapping earlier would
still seem to be possible (and then preferred). In fact
acpi_boot_init() already gets called after vm_init(), so switching
acpi_os_map_memory() to use dynamic mappings when
>= SYS_STATE_boot should already work today (on x86 at least,
not sure about ARM).

Jan


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