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Date: Mon, 05 Sep 2011 12:10:51 +0200
From: Jan Kiszka <jan.kiszka@siemens.com>
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Subject: Re: [Qemu-devel] [PATCH] qemu_vmalloc: align properly for
 transparent hugepages and KVM
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To: Avi Kivity <avi@redhat.com>
Cc: qemu-devel@nongnu.org

On 2011-09-05 10:07, Avi Kivity wrote:
> 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>
> ---
>  oslib-posix.c |   14 +++++++++++++-
>  1 files changed, 13 insertions(+), 1 deletions(-)
> 
> diff --git a/oslib-posix.c b/oslib-posix.c
> index 196099c..a304fb0 100644
> --- a/oslib-posix.c
> +++ b/oslib-posix.c
> @@ -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 */

Aren't transparent hugepages also available in TCG mode? Then just
remove "by KVM" from subject and comment.

Jan

> +#  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 @@ int qemu_daemon(int nochdir, int noclose)
>  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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