[Qemu-devel] [PATCH 1/1] s390: autodetect map private

[Christian Borntraeger]

From: Christian Borntraeger <address@hidden>

kvm on specific s390 systems must not use MAP_PRIVATE since
host read-only page faults are delivered to the guest. Newer
systems allow the distinction via KVM_CAP_S390_COW

Signed-off-by: Christian Borntraeger <address@hidden>
CC: Avi Kivity <address@hidden>
---
 exec.c                    |   54 ++++++++++++++++++++++++++++++++-------------
 kvm-all.c                 |    6 +++++
 kvm.h                     |    1 +
 linux-headers/linux/kvm.h |    1 +
 oslib-posix.c             |    3 +++
 5 files changed, 50 insertions(+), 15 deletions(-)

diff --git a/exec.c b/exec.c
index 0607c9b..68c2940 100644
--- a/exec.c
+++ b/exec.c
@@ -2601,6 +2601,43 @@ void qemu_ram_set_idstr(ram_addr_t addr, const char 
*name, DeviceState *dev)
     }
 }
 
+/*
+ * lets make sure that we dont have the old s390x limitations regarding
+ * guest mappings
+ */
+static int legacy_s390x_mem_layout(void)
+{
+#if defined(TARGET_S390X) && defined(CONFIG_KVM)
+    return kvm_has_legacy_s390x_memlayout();
+#else
+    return 0;
+#endif
+}
+
+/*
+ * 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;
+}
+
 ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
                                    MemoryRegion *mr)
 {
@@ -2627,26 +2664,13 @@ 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 (legacy_s390x_mem_layout()) {
+                new_block->host = legacy_s390_alloc(size);
             } else {
                 new_block->host = qemu_vmalloc(size);
             }
-#endif
             qemu_madvise(new_block->host, size, QEMU_MADV_MERGEABLE);
         }
     }
diff --git a/kvm-all.c b/kvm-all.c
index 9b73ccf..f794546 100644
--- a/kvm-all.c
+++ b/kvm-all.c
@@ -1417,6 +1417,12 @@ int kvm_has_many_ioeventfds(void)
     return kvm_state->many_ioeventfds;
 }
 
+int kvm_has_legacy_s390x_memlayout(void)
+{
+    return !kvm_check_extension(kvm_state, KVM_CAP_S390_GMAP) ||
+           !kvm_check_extension(kvm_state, KVM_CAP_S390_COW);
+}
+
 int kvm_has_gsi_routing(void)
 {
 #ifdef KVM_CAP_IRQ_ROUTING
diff --git a/kvm.h b/kvm.h
index 4ccae8c..37f40ee 100644
--- a/kvm.h
+++ b/kvm.h
@@ -56,6 +56,7 @@ int kvm_has_xsave(void);
 int kvm_has_xcrs(void);
 int kvm_has_pit_state2(void);
 int kvm_has_many_ioeventfds(void);
+int kvm_has_legacy_s390x_memlayout(void);
 int kvm_has_gsi_routing(void);
 
 int kvm_allows_irq0_override(void);
diff --git a/linux-headers/linux/kvm.h b/linux-headers/linux/kvm.h
index ee7bd9c..a7a2d6f 100644
--- a/linux-headers/linux/kvm.h
+++ b/linux-headers/linux/kvm.h
@@ -590,6 +590,7 @@ struct kvm_ppc_pvinfo {
 #define KVM_CAP_SYNC_REGS 74
 #define KVM_CAP_PCI_2_3 75
 #define KVM_CAP_KVMCLOCK_CTRL 76
+#define KVM_CAP_S390_COW 79
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
diff --git a/oslib-posix.c b/oslib-posix.c
index b6a3c7f..93902ac 100644
--- a/oslib-posix.c
+++ b/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
-- 
1.7.10.2