[qemu-s390x] [PATCH v3 1/1] s390-ccw-virtio: allow for systems larger that 7.999TB

[Christian Borntraeger]

KVM does not allow memory regions > KVM_MEM_MAX_NR_PAGES, basically
limiting the memory per slot to 8TB-4k. As memory slots on s390/kvm must
be a multiple of 1MB we need start a new memory region if we cross
8TB-1M.

With that (and optimistic overcommitment in the kernel) I was able to
start a 24TB guest on a 1TB system.

Signed-off-by: Christian Borntraeger <address@hidden>
---
 hw/s390x/s390-virtio-ccw.c | 28 +++++++++++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)

diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
index 8425534..073f6ed 100644
--- a/hw/s390x/s390-virtio-ccw.c
+++ b/hw/s390x/s390-virtio-ccw.c
@@ -154,14 +154,36 @@ static void virtio_ccw_register_hcalls(void)
                                    virtio_ccw_hcall_early_printk);
 }
 
+/*
+ * KVM does only support memory slots up to KVM_MEM_MAX_NR_PAGES pages
+ * as the dirty bitmap must be managed by bitops that take an int as
+ * position indicator. If we have a guest beyond that we will split off
+ * new subregions. The split must happen on a segment boundary (1MB).
+ */
+#define KVM_MEM_MAX_NR_PAGES ((1UL << 31) - 1)
+#define SEG_MSK (~0xfffffULL)
+#define KVM_SLOT_MAX_BYTES ((KVM_MEM_MAX_NR_PAGES * TARGET_PAGE_SIZE) & 
SEG_MSK)
 static void s390_memory_init(ram_addr_t mem_size)
 {
     MemoryRegion *sysmem = get_system_memory();
-    MemoryRegion *ram = g_new(MemoryRegion, 1);
+    ram_addr_t chunk, offset = 0;
+    unsigned int number = 0;
+    gchar *name;
 
     /* allocate RAM for core */
-    memory_region_allocate_system_memory(ram, NULL, "s390.ram", mem_size);
-    memory_region_add_subregion(sysmem, 0, ram);
+    name = g_strdup_printf("s390.ram");
+    while (mem_size) {
+        MemoryRegion *ram = g_new(MemoryRegion, 1);
+        /* KVM does not allow memslots >= 8 TB */
+        chunk = MIN(mem_size, KVM_SLOT_MAX_BYTES);
+        memory_region_allocate_system_memory(ram, NULL, name, chunk);
+        memory_region_add_subregion(sysmem, offset, ram);
+        mem_size -= chunk;
+        offset += chunk;
+        g_free(name);
+        name = g_strdup_printf("s390.ram.%u", ++number);
+    }
+    g_free(name);
 
     /* Initialize storage key device */
     s390_skeys_init();
-- 
2.9.4