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Re: [Qemu-devel] [PATCH] xen-mapcache: use MAP_FIXED flag so the mmap ad
From: |
Anthony PERARD |
Subject: |
Re: [Qemu-devel] [PATCH] xen-mapcache: use MAP_FIXED flag so the mmap address hint is always honored |
Date: |
Fri, 15 Mar 2019 18:14:09 +0000 |
User-agent: |
Mutt/1.11.3 (2019-02-01) |
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 <address@hidden>
The patch looks fine, and MAP_FIXED seems to be the expected behavior
even on Linux.
Acked-by: Anthony PERARD <address@hidden>
> 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