[Qemu-devel] [PATCH 1/6] pseries: Set hash table size based on RAM size

[David Gibson <david@gibson.dropbear.id.au>]

Currently the pseries machine code always attempts to set the size of the
guests's hash page table to 16MB.  However, because of the way the POWER
MMU works, a suitable hash page table size should really depend on memory
size.  16MB will be excessive for guests with <1GB and RAM, and may not be
enough for guests with >2GB of RAM (depending on guest page size and
other factors).

The usual given rule of thumb is that the hash table should be 1/64 of
the size of memory, but in fact the Linux guests we are aiming at don't
really need that much.  This patch, therefore, changes the hash table
allocation code to aim for 1/128 of the size of RAM (rounding up).  When
using KVM, this size may still be adjusted by the host kernel if it is
unable to allocate a suitable (contiguous) table.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

---

This patch can only be safely applied after my earlier patch
"target-ppc: KVM: Fix some kernel version edge cases for
kvmppc_reset_htab()" (already in ppc-next), otherwise the kernel can
end up advertising to the guest a hash table size different from that
actually allocated by the kernel.
---
 hw/spapr.c |   13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/hw/spapr.c b/hw/spapr.c
index 1177efa..a8bd3c1 100644
--- a/hw/spapr.c
+++ b/hw/spapr.c
@@ -725,9 +725,16 @@ static void ppc_spapr_init(ram_addr_t ram_size,
     spapr->fdt_addr = spapr->rtas_addr - FDT_MAX_SIZE;
     load_limit = spapr->fdt_addr - FW_OVERHEAD;
 
-    /* For now, always aim for a 16MB hash table */
-    /* FIXME: we should change this default based on RAM size */
-    spapr->htab_shift = 24;
+    /* We aim for a hash table of size 1/128 the size of RAM.  The
+     * normal rule of thumb is 1/64 the size of RAM, but that's much
+     * more than needed for the Linux guests we support. */
+    spapr->htab_shift = 18; /* Minimum architected size */
+    while (spapr->htab_shift <= 46) {
+        if ((1ULL << (spapr->htab_shift + 7)) >= ram_size) {
+            break;
+        }
+        spapr->htab_shift++;
+    }
 
     /* init CPUs */
     if (cpu_model == NULL) {
-- 
1.7.10.4