Re: [PATCH 8/8] pdx: introduce a command line option for offset compression

[Oleksii Kurochko <oleksii.kurochko@gmail.com>]

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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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