Re: [Qemu-devel] [PATCH QEMU] transparent hugepage support

[Avi Kivity <avi@redhat.com>]

On 03/11/2010 05:14 PM, Andrea Arcangeli wrote:
> From: Andrea Arcangeli<aarcange@redhat.com>
>
> This will allow proper alignment so NPT/EPT can take advantage of linux host
> backing the guest memory with hugepages (only relevant for KVM and not for QEMU
> that has no NPT/EPT support). To complete it, it will also notify
> the kernel that this memory is important to be backed by hugepages with
> madvise (needed for both KVM and QEMU).
>
> Signed-off-by: Andrea Arcangeli<aarcange@redhat.com>
> ---
>
> diff --git a/exec.c b/exec.c
> index 891e0ee..aedd133 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -2628,11 +2628,25 @@ ram_addr_t qemu_ram_alloc(ram_addr_t size)
>                                   PROT_EXEC|PROT_READ|PROT_WRITE,
>                                   MAP_SHARED | MAP_ANONYMOUS, -1, 0);
>   #else
> -        new_block->host = qemu_vmalloc(size);
> -#endif
> +#if defined(__linux__)&&  defined(TARGET_HPAGE_BITS)
> +	if (!kvm_enabled())
> +#endif
> +		new_block->host = qemu_vmalloc(size);
> +#if defined(__linux__)&&  defined(TARGET_HPAGE_BITS)
> +	else
> +		/*
> +		 * Align on HPAGE_SIZE so "(gfn ^ pfn)&
> +		 * (HPAGE_SIZE-1) == 0" to allow KVM to take advantage
> +		 * of hugepages with NPT/EPT.
> +		 */
> +		new_block->host = qemu_memalign(1<<  TARGET_HPAGE_BITS, size);
>    

That is a little wasteful.  How about a hint to mmap() requesting proper 
alignment (MAP_HPAGE_ALIGN)?

Failing that, modify qemu_memalign() to trim excess memory.

Come to think of it, posix_memalign() needs to do that (but doesn't).

> +#endif
>   #ifdef MADV_MERGEABLE
>           madvise(new_block->host, size, MADV_MERGEABLE);
>   #endif
> +#ifdef MADV_HUGEPAGE
> +        madvise(new_block->host, size, MADV_HUGEPAGE);
> +#endif
>       }


-- 
error compiling committee.c: too many arguments to function