s390: autodetect map private

By default qemu will use MAP_PRIVATE for guest pages. This will write
protect pages and thus break on s390 systems that dont support this feature.
Therefore qemu has a hack to always use MAP_SHARED for s390. But MAP_SHARED
has other problems (no dirty pages tracking, a lot more swap overhead etc.)
Newer systems allow the distinction via KVM_CAP_S390_COW. With this feature
qemu can use the standard qemu alloc if available, otherwise it will use
the old s390 hack.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Acked-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Alexander Graf <agraf@suse.de>

exec.c

@@ -2536,26 +2536,14 @@ ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
             exit(1);
 #endif
         } else {
-#if defined(TARGET_S390X) && defined(CONFIG_KVM)
-            /* S390 KVM requires the topmost vma of the RAM to be smaller than
-               an system defined value, which is at least 256GB. Larger systems
-               have larger values. We put the guest between the end of data
-               segment (system break) and this value. We use 32GB as a base to
-               have enough room for the system break to grow. */
-            new_block->host = mmap((void*)0x800000000, size,
-                                   PROT_EXEC|PROT_READ|PROT_WRITE,
-                                   MAP_SHARED | MAP_ANONYMOUS | MAP_FIXED, -1, 0);
-            if (new_block->host == MAP_FAILED) {
-                fprintf(stderr, "Allocating RAM failed\n");
-                abort();
-            }
-#else
             if (xen_enabled()) {
                 xen_ram_alloc(new_block->offset, size, mr);
+            } else if (kvm_enabled()) {
+                /* some s390/kvm configurations have special constraints */
+                new_block->host = kvm_vmalloc(size);
             } else {
                 new_block->host = qemu_vmalloc(size);
             }
-#endif
             qemu_madvise(new_block->host, size, QEMU_MADV_MERGEABLE);
         }
     }

kvm-all.c

@@ -1655,6 +1655,19 @@ int kvm_allows_irq0_override(void)
     return !kvm_irqchip_in_kernel() || kvm_has_gsi_routing();
 }
 
+void *kvm_vmalloc(ram_addr_t size)
+{
+#ifdef TARGET_S390X
+    void *mem;
+
+    mem = kvm_arch_vmalloc(size);
+    if (mem) {
+        return mem;
+    }
+#endif
+    return qemu_vmalloc(size);
+}
+
 void kvm_setup_guest_memory(void *start, size_t size)
 {
     if (!kvm_has_sync_mmu()) {

kvm.h

@@ -70,6 +70,8 @@ int kvm_init_vcpu(CPUArchState *env);
 int kvm_cpu_exec(CPUArchState *env);
 
 #if !defined(CONFIG_USER_ONLY)
+void *kvm_vmalloc(ram_addr_t size);
+void *kvm_arch_vmalloc(ram_addr_t size);
 void kvm_setup_guest_memory(void *start, size_t size);
 
 int kvm_coalesce_mmio_region(target_phys_addr_t start, ram_addr_t size);

oslib-posix.c

@@ -41,6 +41,9 @@ extern int daemon(int, int);
       therefore we need special code which handles running on Valgrind. */
 #  define QEMU_VMALLOC_ALIGN (512 * 4096)
 #  define CONFIG_VALGRIND
+#elif defined(__linux__) && defined(__s390x__)
+   /* Use 1 MiB (segment size) alignment so gmap can be used by KVM. */
+#  define QEMU_VMALLOC_ALIGN (256 * 4096)
 #else
 #  define QEMU_VMALLOC_ALIGN getpagesize()
 #endif

target-s390x/kvm.c

@@ -135,6 +135,41 @@ int kvm_arch_get_registers(CPUS390XState *env)
     return 0;
 }
 
+/*
+ * Legacy layout for s390:
+ * Older S390 KVM requires the topmost vma of the RAM to be
+ * smaller than an system defined value, which is at least 256GB.
+ * Larger systems have larger values. We put the guest between
+ * the end of data segment (system break) and this value. We
+ * use 32GB as a base to have enough room for the system break
+ * to grow. We also have to use MAP parameters that avoid
+ * read-only mapping of guest pages.
+ */
+static void *legacy_s390_alloc(ram_addr_t size)
+{
+    void *mem;
+
+    mem = mmap((void *) 0x800000000ULL, size,
+               PROT_EXEC|PROT_READ|PROT_WRITE,
+               MAP_SHARED | MAP_ANONYMOUS | MAP_FIXED, -1, 0);
+    if (mem == MAP_FAILED) {
+        fprintf(stderr, "Allocating RAM failed\n");
+        abort();
+    }
+    return mem;
+}
+
+void *kvm_arch_vmalloc(ram_addr_t size)
+{
+    /* Can we use the standard allocation ? */
+    if (kvm_check_extension(kvm_state, KVM_CAP_S390_GMAP) &&
+        kvm_check_extension(kvm_state, KVM_CAP_S390_COW)) {
+        return NULL;
+    } else {
+        return legacy_s390_alloc(size);
+    }
+}
+
 int kvm_arch_insert_sw_breakpoint(CPUS390XState *env, struct kvm_sw_breakpoint *bp)
 {
     static const uint8_t diag_501[] = {0x83, 0x24, 0x05, 0x01};