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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 18:38:34 +0000 |
User-agent: |
Mutt/1.11.3 (2019-02-01) |
On Fri, Mar 15, 2019 at 06:14:09PM +0000, Anthony PERARD wrote:
> 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.
> > + */
> > 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);
I've noticed that there's a mmap call just after which has also vaddr as
hint, should that second call also have MAP_FIXED as flags?
On the other hand, it seems that when vaddr!=NULL, dummy==false, so that
second mmap call will never be called with vaddr!=NULL. But it might be
worse fixing this in the same patch.
> } else {
> /*
> * We create dummy mappings where we are unable to create a foreign
> * mapping immediately due to certain circumstances (i.e. on resume now)
> */
> vaddr_base = mmap(vaddr, size, PROT_READ | PROT_WRITE,
> MAP_ANON | MAP_SHARED, -1, 0);
--
Anthony PERARD