Allow controlling whether to attempt PDX compression, and which algorithm
to use to calculate the coefficients. Document the option and also add a
CHANGELOG entry for the newly added feature.
Note the work has been originally done to cope with the new Intel
Sapphire/Granite Rapids, however the compression is not explicitly tied to
Intel or x86, and hence could be helpful on other architectures.
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
CHANGELOG.md | 3 +++
docs/misc/xen-command-line.pandoc | 22 ++++++++++++++++++
xen/common/pdx.c | 38 +++++++++++++++++++++++++++----
3 files changed, 59 insertions(+), 4 deletions(-)
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 23215a8cc1e6..48327f03078f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -20,6 +20,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
table or foreign memory.
### Added
+ - Introduce new PDX compression algorithm to cope with Intel Sapphire and
+ Granite Rapids having sparse memory maps.
+
- On x86:
- Option to attempt to fixup p2m page-faults on PVH dom0.
- Resizable BARs is supported for PVH dom0.
diff --git a/docs/misc/xen-command-line.pandoc b/docs/misc/xen-command-line.pandoc
index b0eadd2c5d58..06819576a06b 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -2072,6 +2072,28 @@ for all of them (`true`), only for those subject to XPTI (`xpti`) or for
those not subject to XPTI (`no-xpti`). The feature is used only in case
INVPCID is supported and not disabled via `invpcid=false`.
+### pdx-compress
+> `= <boolean> | auto | fast | slow`
+
+> Default: `auto`
+
+Only relevant when hypervisor is build with PFN PDX offset compression
+`CONFIG_PDX_OFFSET_COMPRESSION`.
+
+Controls whether Xen will engage in PFN compression, and which algorithm will
+be used to calculate the compression coefficients:
+
+* `auto`: default choice, attempt fast calculation of compression
+ coefficients, if that's not successful fallback to slow calculation.
+
+* `fast`: only attempt fast calculation of coefficients, if it fails PFN PDX
+ compression will be disabled.
+
+* `slow`: only attempt slow calculation of coefficients, if it fails PFN PDX
+ compression will be disabled.
+
+Note `pdx-compress=true` is equivalent to `pdx-compress=auto`.
+
### ple_gap
> `= <integer>`
diff --git a/xen/common/pdx.c b/xen/common/pdx.c
index feabdcded804..5fd01305a7be 100644
--- a/xen/common/pdx.c
+++ b/xen/common/pdx.c
@@ -19,6 +19,7 @@
#include <xen/mm.h>
#include <xen/bitops.h>
#include <xen/nospec.h>
+#include <xen/param.h>
#include <xen/pfn.h>
#include <xen/sections.h>
#include <xen/sort.h>
@@ -468,11 +469,40 @@ STATIC void __init pfn_offset_sanitize_ranges(void)
}
#ifdef __XEN__
+static enum {
+ PDX_AUTO, /* Fast first, fallback to slow if fast is not successful. */
+ PDX_NO, /* Do not attempt compression. */
+ PDX_FAST, /* Only attempt fast calculation of compression parameters. */
+ PDX_SLOW, /* Only attempt slow calculation of compression parameters. */
+} compress_mode __initdata;
+
+static int __init cf_check parse_pdx_param(const char *arg)
+{
+ int val;
+
+ if ( !arg )
+ return -EINVAL;
+
+ if ( (val = parse_bool(arg, NULL)) != -1 )
+ compress_mode = val ? PDX_AUTO : PDX_NO;
+ else if ( !strcmp(arg, "auto") )
+ compress_mode = PDX_AUTO;
+ else if ( !strcmp(arg, "fast") )
+ compress_mode = PDX_FAST;
+ else if ( !strcmp(arg, "slow") )
+ compress_mode = PDX_SLOW;
+ else
+ return -EINVAL;
+
+ return 0;
+}
+custom_param("pdx-compress", parse_pdx_param);
+
bool __init pfn_pdx_compression_setup(paddr_t base)
{
- bool use_slow = false;
+ bool use_slow = compress_mode == PDX_SLOW;
- if ( nr <= 1 )
+ if ( nr <= 1 || compress_mode == PDX_NO )
return false;
if ( nr > ARRAY_SIZE(ranges) )
@@ -507,11 +537,11 @@ bool __init pfn_pdx_compression_setup(paddr_t base)
dprintk(XENLOG_DEBUG,
"Find PFN compression coefficients using %s algorithm failed\n",
use_slow ? "slow" : "fast");
- if ( use_slow )
+ if ( compress_mode == PDX_FAST || use_slow )
return false;
}
- if ( use_slow )
+ if ( compress_mode == PDX_FAST || use_slow )
break;
}
--
2.49.0
On 11.06.2025 19:16, Roger Pau Monne wrote: > --- a/docs/misc/xen-command-line.pandoc > +++ b/docs/misc/xen-command-line.pandoc > @@ -2072,6 +2072,28 @@ for all of them (`true`), only for those subject to XPTI (`xpti`) or for > those not subject to XPTI (`no-xpti`). The feature is used only in case > INVPCID is supported and not disabled via `invpcid=false`. > > +### pdx-compress > +> `= <boolean> | auto | fast | slow` > + > +> Default: `auto` > + > +Only relevant when hypervisor is build with PFN PDX offset compression > +`CONFIG_PDX_OFFSET_COMPRESSION`. Which, however, the name of the command line option doesn't reflect at all. Question is what your thoughts were towards what if another compression method also wanted a command line control. Jan
On Wed, Jun 18, 2025 at 03:36:57PM +0200, Jan Beulich wrote: > On 11.06.2025 19:16, Roger Pau Monne wrote: > > --- a/docs/misc/xen-command-line.pandoc > > +++ b/docs/misc/xen-command-line.pandoc > > @@ -2072,6 +2072,28 @@ for all of them (`true`), only for those subject to XPTI (`xpti`) or for > > those not subject to XPTI (`no-xpti`). The feature is used only in case > > INVPCID is supported and not disabled via `invpcid=false`. > > > > +### pdx-compress > > +> `= <boolean> | auto | fast | slow` > > + > > +> Default: `auto` > > + > > +Only relevant when hypervisor is build with PFN PDX offset compression > > +`CONFIG_PDX_OFFSET_COMPRESSION`. > > Which, however, the name of the command line option doesn't reflect at all. > Question is what your thoughts were towards what if another compression > method also wanted a command line control. This will become a boolean option that applies to all PDX compressions: `pdx-compress = <boolean>` to enabled or disable PDX compression. Thanks, Roger.
On 6/11/25 7:16 PM, Roger Pau Monne wrote:
> Allow controlling whether to attempt PDX compression, and which algorithm
> to use to calculate the coefficients. Document the option and also add a
> CHANGELOG entry for the newly added feature.
>
> Note the work has been originally done to cope with the new Intel
> Sapphire/Granite Rapids, however the compression is not explicitly tied to
> Intel or x86, and hence could be helpful on other architectures.
>
> Signed-off-by: Roger Pau Monné<roger.pau@citrix.com>
> ---
> CHANGELOG.md | 3 +++
> docs/misc/xen-command-line.pandoc | 22 ++++++++++++++++++
> xen/common/pdx.c | 38 +++++++++++++++++++++++++++----
> 3 files changed, 59 insertions(+), 4 deletions(-)
LGTM: Reviewed-by: Oleksii Kurochko<oleksii.kurochko@gmail.com>
Thanks.
~ Oleksii
>
> diff --git a/CHANGELOG.md b/CHANGELOG.md
> index 23215a8cc1e6..48327f03078f 100644
> --- a/CHANGELOG.md
> +++ b/CHANGELOG.md
> @@ -20,6 +20,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
> table or foreign memory.
>
> ### Added
> + - Introduce new PDX compression algorithm to cope with Intel Sapphire and
> + Granite Rapids having sparse memory maps.
> +
> - On x86:
> - Option to attempt to fixup p2m page-faults on PVH dom0.
> - Resizable BARs is supported for PVH dom0.
> diff --git a/docs/misc/xen-command-line.pandoc b/docs/misc/xen-command-line.pandoc
> index b0eadd2c5d58..06819576a06b 100644
> --- a/docs/misc/xen-command-line.pandoc
> +++ b/docs/misc/xen-command-line.pandoc
> @@ -2072,6 +2072,28 @@ for all of them (`true`), only for those subject to XPTI (`xpti`) or for
> those not subject to XPTI (`no-xpti`). The feature is used only in case
> INVPCID is supported and not disabled via `invpcid=false`.
>
> +### pdx-compress
> +> `= <boolean> | auto | fast | slow`
> +
> +> Default: `auto`
> +
> +Only relevant when hypervisor is build with PFN PDX offset compression
> +`CONFIG_PDX_OFFSET_COMPRESSION`.
> +
> +Controls whether Xen will engage in PFN compression, and which algorithm will
> +be used to calculate the compression coefficients:
> +
> +* `auto`: default choice, attempt fast calculation of compression
> + coefficients, if that's not successful fallback to slow calculation.
> +
> +* `fast`: only attempt fast calculation of coefficients, if it fails PFN PDX
> + compression will be disabled.
> +
> +* `slow`: only attempt slow calculation of coefficients, if it fails PFN PDX
> + compression will be disabled.
> +
> +Note `pdx-compress=true` is equivalent to `pdx-compress=auto`.
> +
> ### ple_gap
> > `= <integer>`
>
> diff --git a/xen/common/pdx.c b/xen/common/pdx.c
> index feabdcded804..5fd01305a7be 100644
> --- a/xen/common/pdx.c
> +++ b/xen/common/pdx.c
> @@ -19,6 +19,7 @@
> #include <xen/mm.h>
> #include <xen/bitops.h>
> #include <xen/nospec.h>
> +#include <xen/param.h>
> #include <xen/pfn.h>
> #include <xen/sections.h>
> #include <xen/sort.h>
> @@ -468,11 +469,40 @@ STATIC void __init pfn_offset_sanitize_ranges(void)
> }
>
> #ifdef __XEN__
> +static enum {
> + PDX_AUTO, /* Fast first, fallback to slow if fast is not successful. */
> + PDX_NO, /* Do not attempt compression. */
> + PDX_FAST, /* Only attempt fast calculation of compression parameters. */
> + PDX_SLOW, /* Only attempt slow calculation of compression parameters. */
> +} compress_mode __initdata;
> +
> +static int __init cf_check parse_pdx_param(const char *arg)
> +{
> + int val;
> +
> + if ( !arg )
> + return -EINVAL;
> +
> + if ( (val = parse_bool(arg, NULL)) != -1 )
> + compress_mode = val ? PDX_AUTO : PDX_NO;
> + else if ( !strcmp(arg, "auto") )
> + compress_mode = PDX_AUTO;
> + else if ( !strcmp(arg, "fast") )
> + compress_mode = PDX_FAST;
> + else if ( !strcmp(arg, "slow") )
> + compress_mode = PDX_SLOW;
> + else
> + return -EINVAL;
> +
> + return 0;
> +}
> +custom_param("pdx-compress", parse_pdx_param);
> +
> bool __init pfn_pdx_compression_setup(paddr_t base)
> {
> - bool use_slow = false;
> + bool use_slow = compress_mode == PDX_SLOW;
>
> - if ( nr <= 1 )
> + if ( nr <= 1 || compress_mode == PDX_NO )
> return false;
>
> if ( nr > ARRAY_SIZE(ranges) )
> @@ -507,11 +537,11 @@ bool __init pfn_pdx_compression_setup(paddr_t base)
> dprintk(XENLOG_DEBUG,
> "Find PFN compression coefficients using %s algorithm failed\n",
> use_slow ? "slow" : "fast");
> - if ( use_slow )
> + if ( compress_mode == PDX_FAST || use_slow )
> return false;
> }
>
> - if ( use_slow )
> + if ( compress_mode == PDX_FAST || use_slow )
> break;
> }
>
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