Re: [Xen-devel] [PATCH] xen-mapcache: use MAP_FIXED flag so the mmap address hint is always honored

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Fri, Mar 15, 2019 at 09:58:47AM +0100, Roger Pau Monne wrote:
> Or if it's not possible to honor the hinted address an error is returned
> instead. This makes it easier to spot the actual failure, instead of
> failing later on when the caller of xen_remap_bucket realizes the
> mapping has not been created at the requested address.
> 
> Also note that at least on FreeBSD using MAP_FIXED will cause mmap to
> try harder to honor the passed address.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

The patch looks fine, and MAP_FIXED seems to be the expected behavior
even on Linux.

Acked-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>

> diff --git a/hw/i386/xen/xen-mapcache.c b/hw/i386/xen/xen-mapcache.c
> @@ -185,8 +185,14 @@ static void xen_remap_bucket(MapCacheEntry *entry,
>      }
>  
>      if (!dummy) {
> +        /*
> +         * If the caller has requested the mapping at a specific address use
> +         * MAP_FIXED to make sure it's honored. Note that with MAP_FIXED at
> +         * least FreeBSD will try harder to honor the passed address.
> +         */

I wonder if the note about FreeBSD is actually useful here. Even Linux
may map at a different address without MAP_FIXED, if I read the man page
correctly. Do you mind if it is removed?

>          vaddr_base = xenforeignmemory_map2(xen_fmem, xen_domid, vaddr,
> -                                           PROT_READ | PROT_WRITE, 0,
> +                                           PROT_READ | PROT_WRITE,
> +                                           vaddr ? MAP_FIXED : 0,
>                                             nb_pfn, pfns, err);

Thanks,

-- 
Anthony PERARD

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