[Qemu-devel] Re: [PATCH v4 2/3] Support adding a file to qemu's ram allocation

[Avi Kivity]

On 04/08/2010 01:51 AM, Cam Macdonell wrote:
> This avoids the need of using qemu_ram_alloc and mmap with MAP_FIXED to map a
> host file into guest RAM.  This function mmaps the opened file anywhere and adds
> the memory to the ram blocks.
>
> Usage is
>
> qemu_ram_mmap(fd, size, MAP_SHARED, offset);
> ---
>   cpu-common.h |    1 +
>   exec.c       |   33 +++++++++++++++++++++++++++++++++
>   2 files changed, 34 insertions(+), 0 deletions(-)
>
> diff --git a/cpu-common.h b/cpu-common.h
> index 49c7fb3..87c82fc 100644
> --- a/cpu-common.h
> +++ b/cpu-common.h
> @@ -32,6 +32,7 @@ static inline void 
> cpu_register_physical_memory(target_phys_addr_t start_addr,
>   }
>
>   ram_addr_t cpu_get_physical_page_desc(target_phys_addr_t addr);
> +ram_addr_t qemu_ram_mmap(int, ram_addr_t, int, int);
>    

Use prototypes with argument names please.  That's not the style around 
it, but that's bad style.

> +ram_addr_t qemu_ram_mmap(int fd, ram_addr_t size, int flags, int offset)
>    

off_t offset

> +{
> +    RAMBlock *new_block;
> +
> +    size = TARGET_PAGE_ALIGN(size);
> +    new_block = qemu_malloc(sizeof(*new_block));
> +
> +    // map the file passed as a parameter to be this part of memory
>    

/* comments */

> +    new_block->host = mmap(0, size, PROT_READ|PROT_WRITE, flags, fd, offset);
>    

Error checking.

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

Won't work (ksm only merges anonymous pages), but keep it there in case 
it learns about pagecache.


--
Do not meddle in the internals of kernels, for they are subtle and quick to 
panic.