[PATCH 0/2] vfio: Align mmaps

[PATCH 0/2] vfio: Align mmaps

[Alex Williamson]

As described in patch /2, newer kernels may support pfnmap with PMD or
PUD sized mappings.  Mappings must be aligned in order to see the full
benefit of this support.  We're largely able to get PMD alignment for
free from mmap, but PUD alignment requires some effort.  Further, we
don't actually have an easy way to determine PMD or PUD alignment,
therefore we align all mmaps to the nearest power-of-two relative to the
region size, up to the maximum PUD size known the be currently available.

Enabling debug prints in the kernel shows that this exclusively enables
1GiB mappings for a GPU with a multi-gigabyte BAR whereas previously the
BAR is mapped with a combination of 2MiB and 1GiB mappings, only using
1GiB when opportunistically crossing an alignment boundary.

If there are ways to determine discrete alignment intervals or better
ways to generate a properly aligned address value for mmap, please share.
Thanks,

Alex

Alex Williamson (2):
  vfio/helpers: Refactor vfio_region_mmap() error handling
  vfio/helpers: Align mmaps

 hw/vfio/helpers.c | 66 +++++++++++++++++++++++++++++++++--------------
 1 file changed, 47 insertions(+), 19 deletions(-)

-- 
2.46.2

[PATCH 1/2] vfio/helpers: Refactor vfio_region_mmap() error handling

[Alex Williamson]

Move error handling code to the end of the function so that it can more
easily be shared by new mmap failure conditions.  No functional change
intended.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
---
 hw/vfio/helpers.c | 34 +++++++++++++++++-----------------
 1 file changed, 17 insertions(+), 17 deletions(-)

diff --git a/hw/vfio/helpers.c b/hw/vfio/helpers.c
index ea15c79db0a3..b9e606e364a2 100644
--- a/hw/vfio/helpers.c
+++ b/hw/vfio/helpers.c
@@ -395,7 +395,7 @@ static void vfio_subregion_unmap(VFIORegion *region, int index)
 
 int vfio_region_mmap(VFIORegion *region)
 {
-    int i, prot = 0;
+    int i, ret, prot = 0;
     char *name;
 
     if (!region->mem) {
@@ -411,22 +411,8 @@ int vfio_region_mmap(VFIORegion *region)
                                      region->fd_offset +
                                      region->mmaps[i].offset);
         if (region->mmaps[i].mmap == MAP_FAILED) {
-            int ret = -errno;
-
-            trace_vfio_region_mmap_fault(memory_region_name(region->mem), i,
-                                         region->fd_offset +
-                                         region->mmaps[i].offset,
-                                         region->fd_offset +
-                                         region->mmaps[i].offset +
-                                         region->mmaps[i].size - 1, ret);
-
-            region->mmaps[i].mmap = NULL;
-
-            for (i--; i >= 0; i--) {
-                vfio_subregion_unmap(region, i);
-            }
-
-            return ret;
+            ret = -errno;
+            goto no_mmap;
         }
 
         name = g_strdup_printf("%s mmaps[%d]",
@@ -446,6 +432,20 @@ int vfio_region_mmap(VFIORegion *region)
     }
 
     return 0;
+
+no_mmap:
+    trace_vfio_region_mmap_fault(memory_region_name(region->mem), i,
+                                 region->fd_offset + region->mmaps[i].offset,
+                                 region->fd_offset + region->mmaps[i].offset +
+                                 region->mmaps[i].size - 1, ret);
+
+    region->mmaps[i].mmap = NULL;
+
+    for (i--; i >= 0; i--) {
+        vfio_subregion_unmap(region, i);
+    }
+
+    return ret;
 }
 
 void vfio_region_unmap(VFIORegion *region)
-- 
2.46.2

[PATCH 2/2] vfio/helpers: Align mmaps

[Alex Williamson]

Thanks to work by Peter Xu, support is introduced in Linux v6.12 to
allow pfnmap insertions at PMD and PUD levels of the page table.  This
means that provided a properly aligned mmap, the vfio driver is able
to map MMIO at significantly larger intervals than PAGE_SIZE.  For
example on x86_64 (the only architecture currently supporting huge
pfnmaps for PUD), rather than 4KiB mappings, we can map device MMIO
using 2MiB and even 1GiB page table entries.

Typically mmap will already provide PMD aligned mappings, so devices
with moderately sized MMIO ranges, even GPUs with standard 256MiB BARs,
will already take advantage of this support.  However in order to better
support devices exposing multi-GiB MMIO, such as 3D accelerators or GPUs
with resizable BARs enabled, we need to manually align the mmap.

There doesn't seem to be a way for userspace to easily learn about PMD
and PUD mapping level sizes, therefore this takes the simple approach
to align the mapping to the power-of-two size of the region, up to 1GiB,
which is currently the maximum alignment we care about.

Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
---
 hw/vfio/helpers.c | 32 ++++++++++++++++++++++++++++++--
 1 file changed, 30 insertions(+), 2 deletions(-)

diff --git a/hw/vfio/helpers.c b/hw/vfio/helpers.c
index b9e606e364a2..913796f437f8 100644
--- a/hw/vfio/helpers.c
+++ b/hw/vfio/helpers.c
@@ -27,6 +27,7 @@
 #include "trace.h"
 #include "qapi/error.h"
 #include "qemu/error-report.h"
+#include "qemu/units.h"
 #include "monitor/monitor.h"
 
 /*
@@ -406,8 +407,35 @@ int vfio_region_mmap(VFIORegion *region)
     prot |= region->flags & VFIO_REGION_INFO_FLAG_WRITE ? PROT_WRITE : 0;
 
     for (i = 0; i < region->nr_mmaps; i++) {
-        region->mmaps[i].mmap = mmap(NULL, region->mmaps[i].size, prot,
-                                     MAP_SHARED, region->vbasedev->fd,
+        size_t align = MIN(1ULL << ctz64(region->mmaps[i].size), 1 * GiB);
+        void *map_base, *map_align;
+
+        /*
+         * Align the mmap for more efficient mapping in the kernel.  Ideally
+         * we'd know the PMD and PUD mapping sizes to use as discrete alignment
+         * intervals, but we don't.  As of Linux v6.12, the largest PUD size
+         * supporting huge pfnmap is 1GiB (ARCH_SUPPORTS_PUD_PFNMAP is only set
+         * on x86_64).  Align by power-of-two size, capped at 1GiB.
+         *
+         * NB. qemu_memalign() and friends actually allocate memory, whereas
+         * the region size here can exceed host memory, therefore we manually
+         * create an oversized anonymous mapping and clean it up for alignment.
+         */
+        map_base = mmap(0, region->mmaps[i].size + align, PROT_NONE,
+                        MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+        if (map_base == MAP_FAILED) {
+            ret = -errno;
+            goto no_mmap;
+        }
+
+        map_align = (void *)ROUND_UP((uintptr_t)map_base, (uintptr_t)align);
+        munmap(map_base, map_align - map_base);
+        munmap(map_align + region->mmaps[i].size,
+               align - (map_align - map_base));
+
+        region->mmaps[i].mmap = mmap(map_align, region->mmaps[i].size, prot,
+                                     MAP_SHARED | MAP_FIXED,
+                                     region->vbasedev->fd,
                                      region->fd_offset +
                                      region->mmaps[i].offset);
         if (region->mmaps[i].mmap == MAP_FAILED) {
-- 
2.46.2

Re: [PATCH 1/2] vfio/helpers: Refactor vfio_region_mmap() error handling

[Peter Xu]

On Tue, Oct 22, 2024 at 02:08:28PM -0600, Alex Williamson wrote:
> Move error handling code to the end of the function so that it can more
> easily be shared by new mmap failure conditions.  No functional change
> intended.
> 
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> ---
>  hw/vfio/helpers.c | 34 +++++++++++++++++-----------------
>  1 file changed, 17 insertions(+), 17 deletions(-)
> 
> diff --git a/hw/vfio/helpers.c b/hw/vfio/helpers.c
> index ea15c79db0a3..b9e606e364a2 100644
> --- a/hw/vfio/helpers.c
> +++ b/hw/vfio/helpers.c
> @@ -395,7 +395,7 @@ static void vfio_subregion_unmap(VFIORegion *region, int index)
>  
>  int vfio_region_mmap(VFIORegion *region)
>  {
> -    int i, prot = 0;
> +    int i, ret, prot = 0;
>      char *name;
>  
>      if (!region->mem) {
> @@ -411,22 +411,8 @@ int vfio_region_mmap(VFIORegion *region)
>                                       region->fd_offset +
>                                       region->mmaps[i].offset);
>          if (region->mmaps[i].mmap == MAP_FAILED) {
> -            int ret = -errno;
> -
> -            trace_vfio_region_mmap_fault(memory_region_name(region->mem), i,
> -                                         region->fd_offset +
> -                                         region->mmaps[i].offset,
> -                                         region->fd_offset +
> -                                         region->mmaps[i].offset +
> -                                         region->mmaps[i].size - 1, ret);
> -
> -            region->mmaps[i].mmap = NULL;
> -
> -            for (i--; i >= 0; i--) {
> -                vfio_subregion_unmap(region, i);
> -            }
> -
> -            return ret;
> +            ret = -errno;
> +            goto no_mmap;
>          }
>  
>          name = g_strdup_printf("%s mmaps[%d]",
> @@ -446,6 +432,20 @@ int vfio_region_mmap(VFIORegion *region)
>      }
>  
>      return 0;
> +
> +no_mmap:
> +    trace_vfio_region_mmap_fault(memory_region_name(region->mem), i,
> +                                 region->fd_offset + region->mmaps[i].offset,
> +                                 region->fd_offset + region->mmaps[i].offset +
> +                                 region->mmaps[i].size - 1, ret);

While at it, maybe when moving we can rename s/fault/fail/ or some other
words to avoid "mmap" + "fault" being together. :) Totally not a huge deal.

Reviewed-by: Peter Xu <peterx@redhat.com>

> +
> +    region->mmaps[i].mmap = NULL;
> +
> +    for (i--; i >= 0; i--) {
> +        vfio_subregion_unmap(region, i);
> +    }
> +
> +    return ret;
>  }
>  
>  void vfio_region_unmap(VFIORegion *region)
> -- 
> 2.46.2
> 

-- 
Peter Xu

Re: [PATCH 2/2] vfio/helpers: Align mmaps

[Peter Xu]

On Tue, Oct 22, 2024 at 02:08:29PM -0600, Alex Williamson wrote:
> Thanks to work by Peter Xu, support is introduced in Linux v6.12 to
> allow pfnmap insertions at PMD and PUD levels of the page table.  This
> means that provided a properly aligned mmap, the vfio driver is able
> to map MMIO at significantly larger intervals than PAGE_SIZE.  For
> example on x86_64 (the only architecture currently supporting huge
> pfnmaps for PUD), rather than 4KiB mappings, we can map device MMIO
> using 2MiB and even 1GiB page table entries.
> 
> Typically mmap will already provide PMD aligned mappings, so devices
> with moderately sized MMIO ranges, even GPUs with standard 256MiB BARs,
> will already take advantage of this support.  However in order to better
> support devices exposing multi-GiB MMIO, such as 3D accelerators or GPUs
> with resizable BARs enabled, we need to manually align the mmap.
> 
> There doesn't seem to be a way for userspace to easily learn about PMD
> and PUD mapping level sizes, therefore this takes the simple approach
> to align the mapping to the power-of-two size of the region, up to 1GiB,
> which is currently the maximum alignment we care about.
> 
> Cc: Peter Xu <peterx@redhat.com>
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

For the longer term, maybe QEMU can provide a function to reserve a range
of mmap with some specific alignment requirement.  For example, currently
qemu_ram_mmap() does mostly the same thing (and it hides a hugetlb fix on
ppc only with 7197fb4058, which isn't a concern here).  Then the complexity
can hide in that function.  Kind of a comment for the future only.

Reviewed-by: Peter Xu <peterx@redhat.com>

Thanks!

> ---
>  hw/vfio/helpers.c | 32 ++++++++++++++++++++++++++++++--
>  1 file changed, 30 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/vfio/helpers.c b/hw/vfio/helpers.c
> index b9e606e364a2..913796f437f8 100644
> --- a/hw/vfio/helpers.c
> +++ b/hw/vfio/helpers.c
> @@ -27,6 +27,7 @@
>  #include "trace.h"
>  #include "qapi/error.h"
>  #include "qemu/error-report.h"
> +#include "qemu/units.h"
>  #include "monitor/monitor.h"
>  
>  /*
> @@ -406,8 +407,35 @@ int vfio_region_mmap(VFIORegion *region)
>      prot |= region->flags & VFIO_REGION_INFO_FLAG_WRITE ? PROT_WRITE : 0;
>  
>      for (i = 0; i < region->nr_mmaps; i++) {
> -        region->mmaps[i].mmap = mmap(NULL, region->mmaps[i].size, prot,
> -                                     MAP_SHARED, region->vbasedev->fd,
> +        size_t align = MIN(1ULL << ctz64(region->mmaps[i].size), 1 * GiB);
> +        void *map_base, *map_align;
> +
> +        /*
> +         * Align the mmap for more efficient mapping in the kernel.  Ideally
> +         * we'd know the PMD and PUD mapping sizes to use as discrete alignment
> +         * intervals, but we don't.  As of Linux v6.12, the largest PUD size
> +         * supporting huge pfnmap is 1GiB (ARCH_SUPPORTS_PUD_PFNMAP is only set
> +         * on x86_64).  Align by power-of-two size, capped at 1GiB.
> +         *
> +         * NB. qemu_memalign() and friends actually allocate memory, whereas
> +         * the region size here can exceed host memory, therefore we manually
> +         * create an oversized anonymous mapping and clean it up for alignment.
> +         */
> +        map_base = mmap(0, region->mmaps[i].size + align, PROT_NONE,
> +                        MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> +        if (map_base == MAP_FAILED) {
> +            ret = -errno;
> +            goto no_mmap;
> +        }
> +
> +        map_align = (void *)ROUND_UP((uintptr_t)map_base, (uintptr_t)align);
> +        munmap(map_base, map_align - map_base);
> +        munmap(map_align + region->mmaps[i].size,
> +               align - (map_align - map_base));
> +
> +        region->mmaps[i].mmap = mmap(map_align, region->mmaps[i].size, prot,
> +                                     MAP_SHARED | MAP_FIXED,
> +                                     region->vbasedev->fd,
>                                       region->fd_offset +
>                                       region->mmaps[i].offset);
>          if (region->mmaps[i].mmap == MAP_FAILED) {
> -- 
> 2.46.2
> 

-- 
Peter Xu

Re: [PATCH 1/2] vfio/helpers: Refactor vfio_region_mmap() error handling

[Cédric Le Goater]

On 10/22/24 22:08, Alex Williamson wrote:
> Move error handling code to the end of the function so that it can more
> easily be shared by new mmap failure conditions.  No functional change
> intended.
> 
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>


Reviewed-by: Cédric Le Goater <clg@redhat.com>

Thanks,

C.


> ---
>   hw/vfio/helpers.c | 34 +++++++++++++++++-----------------
>   1 file changed, 17 insertions(+), 17 deletions(-)
> 
> diff --git a/hw/vfio/helpers.c b/hw/vfio/helpers.c
> index ea15c79db0a3..b9e606e364a2 100644
> --- a/hw/vfio/helpers.c
> +++ b/hw/vfio/helpers.c
> @@ -395,7 +395,7 @@ static void vfio_subregion_unmap(VFIORegion *region, int index)
>   
>   int vfio_region_mmap(VFIORegion *region)
>   {
> -    int i, prot = 0;
> +    int i, ret, prot = 0;
>       char *name;
>   
>       if (!region->mem) {
> @@ -411,22 +411,8 @@ int vfio_region_mmap(VFIORegion *region)
>                                        region->fd_offset +
>                                        region->mmaps[i].offset);
>           if (region->mmaps[i].mmap == MAP_FAILED) {
> -            int ret = -errno;
> -
> -            trace_vfio_region_mmap_fault(memory_region_name(region->mem), i,
> -                                         region->fd_offset +
> -                                         region->mmaps[i].offset,
> -                                         region->fd_offset +
> -                                         region->mmaps[i].offset +
> -                                         region->mmaps[i].size - 1, ret);
> -
> -            region->mmaps[i].mmap = NULL;
> -
> -            for (i--; i >= 0; i--) {
> -                vfio_subregion_unmap(region, i);
> -            }
> -
> -            return ret;
> +            ret = -errno;
> +            goto no_mmap;
>           }
>   
>           name = g_strdup_printf("%s mmaps[%d]",
> @@ -446,6 +432,20 @@ int vfio_region_mmap(VFIORegion *region)
>       }
>   
>       return 0;
> +
> +no_mmap:
> +    trace_vfio_region_mmap_fault(memory_region_name(region->mem), i,
> +                                 region->fd_offset + region->mmaps[i].offset,
> +                                 region->fd_offset + region->mmaps[i].offset +
> +                                 region->mmaps[i].size - 1, ret);
> +
> +    region->mmaps[i].mmap = NULL;
> +
> +    for (i--; i >= 0; i--) {
> +        vfio_subregion_unmap(region, i);
> +    }
> +
> +    return ret;
>   }
>   
>   void vfio_region_unmap(VFIORegion *region)

Re: [PATCH 2/2] vfio/helpers: Align mmaps

[Cédric Le Goater]

On 10/22/24 22:08, Alex Williamson wrote:
> Thanks to work by Peter Xu, support is introduced in Linux v6.12 to
> allow pfnmap insertions at PMD and PUD levels of the page table.  This
> means that provided a properly aligned mmap, the vfio driver is able
> to map MMIO at significantly larger intervals than PAGE_SIZE.  For
> example on x86_64 (the only architecture currently supporting huge
> pfnmaps for PUD), rather than 4KiB mappings, we can map device MMIO
> using 2MiB and even 1GiB page table entries.
> 
> Typically mmap will already provide PMD aligned mappings, so devices
> with moderately sized MMIO ranges, even GPUs with standard 256MiB BARs,
> will already take advantage of this support.  However in order to better
> support devices exposing multi-GiB MMIO, such as 3D accelerators or GPUs
> with resizable BARs enabled, we need to manually align the mmap.
> 
> There doesn't seem to be a way for userspace to easily learn about PMD
> and PUD mapping level sizes, therefore this takes the simple approach
> to align the mapping to the power-of-two size of the region, up to 1GiB,
> which is currently the maximum alignment we care about.


Couldn't we inspect /sys/kernel/mm/hugepages/ to get the sizes ?


> Cc: Peter Xu <peterx@redhat.com>
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

anyhow,


Reviewed-by: Cédric Le Goater <clg@redhat.com>

Thanks,

C.



> ---
>   hw/vfio/helpers.c | 32 ++++++++++++++++++++++++++++++--
>   1 file changed, 30 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/vfio/helpers.c b/hw/vfio/helpers.c
> index b9e606e364a2..913796f437f8 100644
> --- a/hw/vfio/helpers.c
> +++ b/hw/vfio/helpers.c
> @@ -27,6 +27,7 @@
>   #include "trace.h"
>   #include "qapi/error.h"
>   #include "qemu/error-report.h"
> +#include "qemu/units.h"
>   #include "monitor/monitor.h"
>   
>   /*
> @@ -406,8 +407,35 @@ int vfio_region_mmap(VFIORegion *region)
>       prot |= region->flags & VFIO_REGION_INFO_FLAG_WRITE ? PROT_WRITE : 0;
>   
>       for (i = 0; i < region->nr_mmaps; i++) {
> -        region->mmaps[i].mmap = mmap(NULL, region->mmaps[i].size, prot,
> -                                     MAP_SHARED, region->vbasedev->fd,
> +        size_t align = MIN(1ULL << ctz64(region->mmaps[i].size), 1 * GiB);
> +        void *map_base, *map_align;
> +
> +        /*
> +         * Align the mmap for more efficient mapping in the kernel.  Ideally
> +         * we'd know the PMD and PUD mapping sizes to use as discrete alignment
> +         * intervals, but we don't.  As of Linux v6.12, the largest PUD size
> +         * supporting huge pfnmap is 1GiB (ARCH_SUPPORTS_PUD_PFNMAP is only set
> +         * on x86_64).  Align by power-of-two size, capped at 1GiB.
> +         *
> +         * NB. qemu_memalign() and friends actually allocate memory, whereas
> +         * the region size here can exceed host memory, therefore we manually
> +         * create an oversized anonymous mapping and clean it up for alignment.
> +         */
> +        map_base = mmap(0, region->mmaps[i].size + align, PROT_NONE,
> +                        MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> +        if (map_base == MAP_FAILED) {
> +            ret = -errno;
> +            goto no_mmap;
> +        }
> +
> +        map_align = (void *)ROUND_UP((uintptr_t)map_base, (uintptr_t)align);
> +        munmap(map_base, map_align - map_base);
> +        munmap(map_align + region->mmaps[i].size,
> +               align - (map_align - map_base));
> +
> +        region->mmaps[i].mmap = mmap(map_align, region->mmaps[i].size, prot,
> +                                     MAP_SHARED | MAP_FIXED,
> +                                     region->vbasedev->fd,
>                                        region->fd_offset +
>                                        region->mmaps[i].offset);
>           if (region->mmaps[i].mmap == MAP_FAILED) {

Re: [PATCH 2/2] vfio/helpers: Align mmaps

[Alex Williamson]

On Wed, 23 Oct 2024 14:44:19 +0200
Cédric Le Goater <clg@redhat.com> wrote:

> On 10/22/24 22:08, Alex Williamson wrote:
> > Thanks to work by Peter Xu, support is introduced in Linux v6.12 to
> > allow pfnmap insertions at PMD and PUD levels of the page table.  This
> > means that provided a properly aligned mmap, the vfio driver is able
> > to map MMIO at significantly larger intervals than PAGE_SIZE.  For
> > example on x86_64 (the only architecture currently supporting huge
> > pfnmaps for PUD), rather than 4KiB mappings, we can map device MMIO
> > using 2MiB and even 1GiB page table entries.
> > 
> > Typically mmap will already provide PMD aligned mappings, so devices
> > with moderately sized MMIO ranges, even GPUs with standard 256MiB BARs,
> > will already take advantage of this support.  However in order to better
> > support devices exposing multi-GiB MMIO, such as 3D accelerators or GPUs
> > with resizable BARs enabled, we need to manually align the mmap.
> > 
> > There doesn't seem to be a way for userspace to easily learn about PMD
> > and PUD mapping level sizes, therefore this takes the simple approach
> > to align the mapping to the power-of-two size of the region, up to 1GiB,
> > which is currently the maximum alignment we care about.  
> 
> 
> Couldn't we inspect /sys/kernel/mm/hugepages/ to get the sizes ?

Sifting through sysfs doesn't seem like a great solution if we want to
support running QEMU in a sandbox with limited access, but also
hugepage sizes don't seem to strictly map to PMD and PUD page table
levels on all platforms.  For instance ARM seems to support an
assortment of hugepage sizes, some of which appear to be available via
contiguous page hinting rather than actual page table level sizes.  At
that point we're still approximating the actual discrete mapping
intervals, but at a substantial increase in complexity, unless we
already have dependencies and existing code that can be leveraged.

> > Cc: Peter Xu <peterx@redhat.com>
> > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>  
> 
> anyhow,
> 
> 
> Reviewed-by: Cédric Le Goater <clg@redhat.com>

Thanks!

Alex
 
> > ---
> >   hw/vfio/helpers.c | 32 ++++++++++++++++++++++++++++++--
> >   1 file changed, 30 insertions(+), 2 deletions(-)
> > 
> > diff --git a/hw/vfio/helpers.c b/hw/vfio/helpers.c
> > index b9e606e364a2..913796f437f8 100644
> > --- a/hw/vfio/helpers.c
> > +++ b/hw/vfio/helpers.c
> > @@ -27,6 +27,7 @@
> >   #include "trace.h"
> >   #include "qapi/error.h"
> >   #include "qemu/error-report.h"
> > +#include "qemu/units.h"
> >   #include "monitor/monitor.h"
> >   
> >   /*
> > @@ -406,8 +407,35 @@ int vfio_region_mmap(VFIORegion *region)
> >       prot |= region->flags & VFIO_REGION_INFO_FLAG_WRITE ? PROT_WRITE : 0;
> >   
> >       for (i = 0; i < region->nr_mmaps; i++) {
> > -        region->mmaps[i].mmap = mmap(NULL, region->mmaps[i].size, prot,
> > -                                     MAP_SHARED, region->vbasedev->fd,
> > +        size_t align = MIN(1ULL << ctz64(region->mmaps[i].size), 1 * GiB);
> > +        void *map_base, *map_align;
> > +
> > +        /*
> > +         * Align the mmap for more efficient mapping in the kernel.  Ideally
> > +         * we'd know the PMD and PUD mapping sizes to use as discrete alignment
> > +         * intervals, but we don't.  As of Linux v6.12, the largest PUD size
> > +         * supporting huge pfnmap is 1GiB (ARCH_SUPPORTS_PUD_PFNMAP is only set
> > +         * on x86_64).  Align by power-of-two size, capped at 1GiB.
> > +         *
> > +         * NB. qemu_memalign() and friends actually allocate memory, whereas
> > +         * the region size here can exceed host memory, therefore we manually
> > +         * create an oversized anonymous mapping and clean it up for alignment.
> > +         */
> > +        map_base = mmap(0, region->mmaps[i].size + align, PROT_NONE,
> > +                        MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> > +        if (map_base == MAP_FAILED) {
> > +            ret = -errno;
> > +            goto no_mmap;
> > +        }
> > +
> > +        map_align = (void *)ROUND_UP((uintptr_t)map_base, (uintptr_t)align);
> > +        munmap(map_base, map_align - map_base);
> > +        munmap(map_align + region->mmaps[i].size,
> > +               align - (map_align - map_base));
> > +
> > +        region->mmaps[i].mmap = mmap(map_align, region->mmaps[i].size, prot,
> > +                                     MAP_SHARED | MAP_FIXED,
> > +                                     region->vbasedev->fd,
> >                                        region->fd_offset +
> >                                        region->mmaps[i].offset);
> >           if (region->mmaps[i].mmap == MAP_FAILED) {  
> 

Re: [PATCH 0/2] vfio: Align mmaps

[Cédric Le Goater]

On 10/22/24 22:08, Alex Williamson wrote:
> As described in patch /2, newer kernels may support pfnmap with PMD or
> PUD sized mappings.  Mappings must be aligned in order to see the full
> benefit of this support.  We're largely able to get PMD alignment for
> free from mmap, but PUD alignment requires some effort.  Further, we
> don't actually have an easy way to determine PMD or PUD alignment,
> therefore we align all mmaps to the nearest power-of-two relative to the
> region size, up to the maximum PUD size known the be currently available.
> 
> Enabling debug prints in the kernel shows that this exclusively enables
> 1GiB mappings for a GPU with a multi-gigabyte BAR whereas previously the
> BAR is mapped with a combination of 2MiB and 1GiB mappings, only using
> 1GiB when opportunistically crossing an alignment boundary.
> 
> If there are ways to determine discrete alignment intervals or better
> ways to generate a properly aligned address value for mmap, please share.
> Thanks,
> 
> Alex
> 
> Alex Williamson (2):
>    vfio/helpers: Refactor vfio_region_mmap() error handling
>    vfio/helpers: Align mmaps
> 
>   hw/vfio/helpers.c | 66 +++++++++++++++++++++++++++++++++--------------
>   1 file changed, 47 insertions(+), 19 deletions(-)
> 


Applied to vfio-next.

Thanks,

C.