acpi: enlarge NUM_FIXMAP_ACPI_PAGES to support larger scale boards

In acpi_tb_verify_table()->__acpi_map_table(), it suppose all ACPI tables
may not exceed 4 pages, the tables includes SRAT/APIC/ERST etc.
Please note that the table DSDT is not mapped through
acpi_tb_verify_table(), thus we don't care its size although it's usually
the largest table among all the ACPI tables. Then the biggest table we
concern is SRAT.
As we know, the size of SRAT if affected by both CPU number and memory
slot number, each CPU costs 24B, and each memory slot costs 40B.

Please note: even when SRAT table is within 4 pages, eg. 14128B, in
__acpi_map_table(), it maps pages to get the table. suppose the start
address is near the end of the first page:

       1000B    4096B         4096B          4096B      840B
       |___|_____________|______________|______________|____|

although the total page is within 4 pages , but it may be in fact across 5
pages, as shown above. Thus the NUM_FIXMAP_ACPI_PAGES should be much
larger nowadays. If not, xen would wrongly thinks no NUMA configuration
could be found as that it could not get SRAT table.

Thus, we make NUM_FIXMAP_ACPI_PAGES much larger, to 64(256KB). it's
calculated for that the theoretical largest CPU number on main Linux
distros is about 8192, and memory slots number should be within 1000,
that's 24B*8192+40B*1000 = 236608B. Meanwhile, because IOREMAP_VIRT_*
region is 16GB, thus I think extending it to 256KB is safe enough.

Of course, there's much more work to do to support large scale boards of
that many(8192) CPUs and 1000 memory slots. We just make life easier for
boards with serveral hundreds of CPUs and serveral TBs of memory.

Signed-off-by: Zhang Bo <oscar.zhangbo@huawei.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/include/xen/acpi.h b/xen/include/xen/acpi.h
index 30ec0eec5f6dff1d32d153f509031b7074f09208..9409350f05d80601dc87ca64dcb8c7a1c8ec28b3 100644 (file)
--- a/xen/include/xen/acpi.h
+++ b/xen/include/xen/acpi.h
@@ -41,9 +41,9 @@
 
 /*
  * Fixmap pages to reserve for ACPI boot-time tables (see asm-x86/fixmap.h or
- * asm-arm/config.h)
+ * asm-arm/config.h, 64 pages(256KB) is large enough for most cases.)
  */
-#define NUM_FIXMAP_ACPI_PAGES  4
+#define NUM_FIXMAP_ACPI_PAGES  64
 
 #define BAD_MADT_ENTRY(entry, end) (                                        \
                 (!(entry)) || (unsigned long)(entry) + sizeof(*(entry)) > (end) ||  \