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

[Paul Durrant]

> -----Original Message-----
> From: Roger Pau Monne [mailto:address@hidden
> Sent: 15 March 2019 08:59
> To: address@hidden
> Cc: Roger Pau Monne <address@hidden>; Stefano Stabellini <address@hidden>; 
> Anthony
> Perard <address@hidden>; Paul Durrant <address@hidden>; Igor Druzhinin
> <address@hidden>; Paolo Bonzini <address@hidden>; Richard Henderson 
> <address@hidden>;
> Eduardo Habkost <address@hidden>; Michael S. Tsirkin <address@hidden>; Marcel 
> Apfelbaum
> <address@hidden>; address@hidden
> Subject: [PATCH] xen-mapcache: use MAP_FIXED flag so the mmap address hint is 
> always honored
> 
> 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 <address@hidden>
> ---
> Cc: Stefano Stabellini <address@hidden>
> Cc: Anthony Perard <address@hidden>
> Cc: Paul Durrant <address@hidden>
> Cc: Igor Druzhinin <address@hidden>
> Cc: Paolo Bonzini <address@hidden>
> Cc: Richard Henderson <address@hidden>
> Cc: Eduardo Habkost <address@hidden>
> Cc: "Michael S. Tsirkin" <address@hidden>
> Cc: Marcel Apfelbaum <address@hidden>
> Cc: address@hidden
> ---
>  hw/i386/xen/xen-mapcache.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/i386/xen/xen-mapcache.c b/hw/i386/xen/xen-mapcache.c
> index 349f72d00c..0e2870b320 100644
> --- 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.
> +         */
>          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);

AFAICT xen_remap_bucket() is always called with a NULL vaddr argument if 
entry->vaddr_base == NULL, and called with vaddr == entry->vaddr_base in the 
other case, so I'd say the vaddr argument is superfluous.
So, I think it would be better to just use entry->vaddr_base in the call above 
and I also wonder whether the mmap() below should only be allowed to happen if 
entry->vaddr_base == NULL.

  Paul

>          if (vaddr_base == NULL) {
>              perror("xenforeignmemory_map2");
> --
> 2.17.2 (Apple Git-113)