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[Qemu-devel] [PATCH] kvm: Avoid COW if KVM MMU is asynchronous

From: Jan Kiszka
Subject: [Qemu-devel] [PATCH] kvm: Avoid COW if KVM MMU is asynchronous
Date: Sun, 26 Apr 2009 14:21:13 +0200
User-agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv: Gecko/20080226 SUSE/ Thunderbird/ Mnenhy/

If the KVM MMU is asynchronous (kernel does not support MMU_NOTIFIER),
we have to avoid COW for the guest memory. Otherwise we risk serious
breakage when guest pages change there physical locations due to COW
after fork. Seen when forking smbd during runtime via -smb.

Signed-off-by: Jan Kiszka <address@hidden>

 exec.c |   14 ++++++++++++++
 1 files changed, 14 insertions(+), 0 deletions(-)

diff --git a/exec.c b/exec.c
index 23013fb..ce0603e 100644
--- a/exec.c
+++ b/exec.c
@@ -2477,6 +2477,20 @@ ram_addr_t qemu_ram_alloc(ram_addr_t size)
     new_block->next = ram_blocks;
     ram_blocks = new_block;
+    if (kvm_enabled() && !kvm_has_sync_mmu()) {
+        int ret = madvise(new_block->host, size, MADV_DONTFORK);
+        if (ret) {
+            perror("madvice");
+            exit(1);
+        }
+        fprintf(stderr,
+                "Need MADV_DONTFORK in absence of synchronous KVM MMU\n");
+        exit(1);
+    }
     phys_ram_dirty = qemu_realloc(phys_ram_dirty,
         (last_ram_offset + size) >> TARGET_PAGE_BITS);
     memset(phys_ram_dirty + (last_ram_offset >> TARGET_PAGE_BITS),

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