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qemu_vmalloc: align properly for transparent hugepages and KVM

To make good use of transparent hugepages, KVM requires that guest-physical
and host-virtual addresses share the low 21 bits (as opposed to just the low
12 bits normally required).

Adjust qemu_vmalloc() to honor that requirement.  Ignore it for small regions
to avoid fragmentation.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
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1 parent df00bed commit 36b586284e678da28df3af9fd0907d2b16f9311c Avi Kivity committed with Anthony Liguori Sep 5, 2011
Showing with 13 additions and 1 deletion.
  1. +13 −1 oslib-posix.c
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@@ -35,6 +35,13 @@
extern int daemon(int, int);
#endif
+#if defined(__linux__) && defined(__x86_64__)
+ /* Use 2MB alignment so transparent hugepages can be used by KVM */
+# define QEMU_VMALLOC_ALIGN (512 * 4096)
+#else
+# define QEMU_VMALLOC_ALIGN getpagesize()
+#endif
+
#include "config-host.h"
#include "sysemu.h"
#include "trace.h"
@@ -80,7 +87,12 @@ void *qemu_memalign(size_t alignment, size_t size)
void *qemu_vmalloc(size_t size)
{
void *ptr;
- ptr = qemu_memalign(getpagesize(), size);
+ size_t align = QEMU_VMALLOC_ALIGN;
+
+ if (size < align) {
+ align = getpagesize();
+ }
+ ptr = qemu_memalign(align, size);
trace_qemu_vmalloc(size, ptr);
return ptr;
}

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