diff mbox series

[v2] xen-mapcache: use MAP_FIXED flag so the mmap address hint is always honored

Message ID 20190318154545.13974-1-roger.pau@citrix.com (mailing list archive)
State Superseded
Headers show
Series [v2] xen-mapcache: use MAP_FIXED flag so the mmap address hint is always honored | expand

Commit Message

Roger Pau Monné March 18, 2019, 3:45 p.m. UTC 
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@citrix.com>
---
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Paul Durrant <paul.durrant@citrix.com>
Cc: Igor Druzhinin <igor.druzhinin@citrix.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Cc: xen-devel@lists.xenproject.org
---
Changes since v1:
 - Use MAP_FIXED for the dummy mmap call also if a specific virtual
   address is requested.
---
 hw/i386/xen/xen-mapcache.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

Comments

Paul Durrant March 18, 2019, 3:47 p.m. UTC | #1 
> -----Original Message-----
> From: Roger Pau Monne [mailto:roger.pau@citrix.com]
> Sent: 18 March 2019 15:46
> To: qemu-devel@nongnu.org
> Cc: Roger Pau Monne <roger.pau@citrix.com>; Stefano Stabellini <sstabellini@kernel.org>; Anthony
> Perard <anthony.perard@citrix.com>; Paul Durrant <Paul.Durrant@citrix.com>; Igor Druzhinin
> <igor.druzhinin@citrix.com>; Paolo Bonzini <pbonzini@redhat.com>; Richard Henderson <rth@twiddle.net>;
> Eduardo Habkost <ehabkost@redhat.com>; Michael S. Tsirkin <mst@redhat.com>; Marcel Apfelbaum
> <marcel.apfelbaum@gmail.com>; xen-devel@lists.xenproject.org
> Subject: [PATCH v2] 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 <roger.pau@citrix.com>

Reviewed-by: Paul Durrant <paul.durrant@citrix.com>

> ---
> Cc: Stefano Stabellini <sstabellini@kernel.org>
> Cc: Anthony Perard <anthony.perard@citrix.com>
> Cc: Paul Durrant <paul.durrant@citrix.com>
> Cc: Igor Druzhinin <igor.druzhinin@citrix.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Richard Henderson <rth@twiddle.net>
> Cc: Eduardo Habkost <ehabkost@redhat.com>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
> Cc: xen-devel@lists.xenproject.org
> ---
> Changes since v1:
>  - Use MAP_FIXED for the dummy mmap call also if a specific virtual
>    address is requested.
> ---
>  hw/i386/xen/xen-mapcache.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/i386/xen/xen-mapcache.c b/hw/i386/xen/xen-mapcache.c
> index 349f72d00c..23de5517db 100644
> --- a/hw/i386/xen/xen-mapcache.c
> +++ b/hw/i386/xen/xen-mapcache.c
> @@ -185,8 +185,13 @@ 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.
> +         */
>          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);
>          if (vaddr_base == NULL) {
>              perror("xenforeignmemory_map2");
> @@ -198,7 +203,8 @@ static void xen_remap_bucket(MapCacheEntry *entry,
>           * 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);
> +                          MAP_ANON | MAP_SHARED | (vaddr ? MAP_FIXED : 0),
> +                          -1, 0);
>          if (vaddr_base == MAP_FAILED) {
>              perror("mmap");
>              exit(-1);
> --
> 2.17.2 (Apple Git-113)
Igor Druzhinin March 18, 2019, 3:48 p.m. UTC | #2 
On 18/03/2019 15:45, 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@citrix.com>
> ---
> Cc: Stefano Stabellini <sstabellini@kernel.org>
> Cc: Anthony Perard <anthony.perard@citrix.com>
> Cc: Paul Durrant <paul.durrant@citrix.com>
> Cc: Igor Druzhinin <igor.druzhinin@citrix.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Richard Henderson <rth@twiddle.net>
> Cc: Eduardo Habkost <ehabkost@redhat.com>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
> Cc: xen-devel@lists.xenproject.org
> ---
> Changes since v1:
>  - Use MAP_FIXED for the dummy mmap call also if a specific virtual
>    address is requested.
> ---
>  hw/i386/xen/xen-mapcache.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/i386/xen/xen-mapcache.c b/hw/i386/xen/xen-mapcache.c
> index 349f72d00c..23de5517db 100644
> --- a/hw/i386/xen/xen-mapcache.c
> +++ b/hw/i386/xen/xen-mapcache.c
> @@ -185,8 +185,13 @@ 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.
> +         */

Since the comment now applied to both invocation - could it be moved
outside the if statement then?

Igor
Roger Pau Monné March 18, 2019, 4:03 p.m. UTC | #3 
On Mon, Mar 18, 2019 at 03:48:59PM +0000, Igor Druzhinin wrote:
> On 18/03/2019 15:45, 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@citrix.com>
> > ---
> > Cc: Stefano Stabellini <sstabellini@kernel.org>
> > Cc: Anthony Perard <anthony.perard@citrix.com>
> > Cc: Paul Durrant <paul.durrant@citrix.com>
> > Cc: Igor Druzhinin <igor.druzhinin@citrix.com>
> > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > Cc: Richard Henderson <rth@twiddle.net>
> > Cc: Eduardo Habkost <ehabkost@redhat.com>
> > Cc: "Michael S. Tsirkin" <mst@redhat.com>
> > Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
> > Cc: xen-devel@lists.xenproject.org
> > ---
> > Changes since v1:
> >  - Use MAP_FIXED for the dummy mmap call also if a specific virtual
> >    address is requested.
> > ---
> >  hw/i386/xen/xen-mapcache.c | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/hw/i386/xen/xen-mapcache.c b/hw/i386/xen/xen-mapcache.c
> > index 349f72d00c..23de5517db 100644
> > --- a/hw/i386/xen/xen-mapcache.c
> > +++ b/hw/i386/xen/xen-mapcache.c
> > @@ -185,8 +185,13 @@ 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.
> > +         */
> 
> Since the comment now applied to both invocation - could it be moved
> outside the if statement then?

I felt that was too far from the actual call, but I can move it if
there's consensus.

Thanks, Roger.
Anthony PERARD March 18, 2019, 4:35 p.m. UTC | #4 
On Mon, Mar 18, 2019 at 03:48:59PM +0000, Igor Druzhinin wrote:
> On 18/03/2019 15:45, Roger Pau Monne wrote:
> > diff --git a/hw/i386/xen/xen-mapcache.c b/hw/i386/xen/xen-mapcache.c
> > index 349f72d00c..23de5517db 100644
> > --- a/hw/i386/xen/xen-mapcache.c
> > +++ b/hw/i386/xen/xen-mapcache.c
> > @@ -185,8 +185,13 @@ 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.
> > +         */
> 
> Since the comment now applied to both invocation - could it be moved
> outside the if statement then?

That sounds good to me.
Paul Durrant March 18, 2019, 4:36 p.m. UTC | #5 
> -----Original Message-----
> From: Anthony PERARD [mailto:anthony.perard@citrix.com]
> Sent: 18 March 2019 16:35
> To: Igor Druzhinin <igor.druzhinin@citrix.com>
> Cc: Roger Pau Monne <roger.pau@citrix.com>; qemu-devel@nongnu.org; Stefano Stabellini
> <sstabellini@kernel.org>; Paul Durrant <Paul.Durrant@citrix.com>; Paolo Bonzini <pbonzini@redhat.com>;
> Richard Henderson <rth@twiddle.net>; Eduardo Habkost <ehabkost@redhat.com>; Michael S. Tsirkin
> <mst@redhat.com>; Marcel Apfelbaum <marcel.apfelbaum@gmail.com>; xen-devel@lists.xenproject.org
> Subject: Re: [PATCH v2] xen-mapcache: use MAP_FIXED flag so the mmap address hint is always honored
> 
> On Mon, Mar 18, 2019 at 03:48:59PM +0000, Igor Druzhinin wrote:
> > On 18/03/2019 15:45, Roger Pau Monne wrote:
> > > diff --git a/hw/i386/xen/xen-mapcache.c b/hw/i386/xen/xen-mapcache.c
> > > index 349f72d00c..23de5517db 100644
> > > --- a/hw/i386/xen/xen-mapcache.c
> > > +++ b/hw/i386/xen/xen-mapcache.c
> > > @@ -185,8 +185,13 @@ 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.
> > > +         */
> >
> > Since the comment now applied to both invocation - could it be moved
> > outside the if statement then?
> 
> That sounds good to me.

I have no problem and my R-b still stands.

  Paul

> 
> --
> Anthony PERARD
diff mbox series

Patch

diff --git a/hw/i386/xen/xen-mapcache.c b/hw/i386/xen/xen-mapcache.c
index 349f72d00c..23de5517db 100644
--- a/hw/i386/xen/xen-mapcache.c
+++ b/hw/i386/xen/xen-mapcache.c
@@ -185,8 +185,13 @@  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.
+         */
         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);
         if (vaddr_base == NULL) {
             perror("xenforeignmemory_map2");
@@ -198,7 +203,8 @@  static void xen_remap_bucket(MapCacheEntry *entry,
          * 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);
+                          MAP_ANON | MAP_SHARED | (vaddr ? MAP_FIXED : 0),
+                          -1, 0);
         if (vaddr_base == MAP_FAILED) {
             perror("mmap");
             exit(-1);