docs/man/xl.cfg.5.pod.in | 16 ++++++++++++ tools/include/libxl.h | 8 ++++++ tools/include/xenguest.h | 4 +++ tools/libs/guest/xg_dom_x86.c | 42 ++++++++++++++++++++++++++++++++ tools/libs/light/libxl_types.idl | 1 + tools/libs/light/libxl_x86.c | 4 +++ tools/xl/xl_parse.c | 11 +++++++++ 7 files changed, 86 insertions(+)
Introduce a new option to start the BSP vCPU in x2APIC mode instead
of xAPIC mode. Expose this in xl through a new "x2apic_mode" option.
Signed-off-by: Teddy Astie <teddy.astie@vates.tech>
---
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Alejandro Vallejo <alejandro.garciavallejo@amd.com>
Cc: Grygorii Strashko <grygorii_strashko@epam.com>
Later on, we could consider with this option to use x2APIC ACPI
tables instead of xAPIC ones.
There is also some room into introducing a new Kconfig option to
only support x2apic mode, which would change how the "Xen default"
would behave.
changed in v2:
- only pre-enable instead of forcing
- use domain builder to pre-enable instead of introducing a new domain creation flag
v1:
- https://lore.kernel.org/xen-devel/d498a50f6187b362ac5da3c6a7a7c348f35dc4b3.1761761288.git.teddy.astie@vates.tech/
---
docs/man/xl.cfg.5.pod.in | 16 ++++++++++++
tools/include/libxl.h | 8 ++++++
tools/include/xenguest.h | 4 +++
tools/libs/guest/xg_dom_x86.c | 42 ++++++++++++++++++++++++++++++++
tools/libs/light/libxl_types.idl | 1 +
tools/libs/light/libxl_x86.c | 4 +++
tools/xl/xl_parse.c | 11 +++++++++
7 files changed, 86 insertions(+)
diff --git a/docs/man/xl.cfg.5.pod.in b/docs/man/xl.cfg.5.pod.in
index ad1553c5e9..0f7a89fe92 100644
--- a/docs/man/xl.cfg.5.pod.in
+++ b/docs/man/xl.cfg.5.pod.in
@@ -3198,6 +3198,22 @@ option.
If using this option is necessary to fix an issue, please report a bug.
+=item B<x2apic_mode="MODE">
+
+Sets the x2apic mode of the domain. The valid values are as follows:
+
+=over 4
+
+=item B<"default">
+
+Use default Xen LAPIC behavior.
+
+=item B<"pre_enable">
+
+Initially enable x2apic for the BSP of the domain.
+
+=back
+
=back
=head1 SEE ALSO
diff --git a/tools/include/libxl.h b/tools/include/libxl.h
index bc35e412da..9850e8aa41 100644
--- a/tools/include/libxl.h
+++ b/tools/include/libxl.h
@@ -1537,6 +1537,14 @@ void libxl_mac_copy(libxl_ctx *ctx, libxl_mac *dst, const libxl_mac *src);
*/
#define LIBXL_HAVE_XEN_PLATFORM_PCI_BAR_UC
+/*
+ * LIBXL_HAVE_X2APIC_PREENABLE
+ *
+ * libxl_domain_build_info contains a boolean 'arch_x86.x2apic_preenable' field
+ * to initially set the BSP LAPIC in x2APIC mode.
+ */
+#define LIBXL_HAVE_X2APIC_PREENABLE
+
typedef char **libxl_string_list;
void libxl_string_list_dispose(libxl_string_list *sl);
int libxl_string_list_length(const libxl_string_list *sl);
diff --git a/tools/include/xenguest.h b/tools/include/xenguest.h
index c88958faa9..408a0c77e8 100644
--- a/tools/include/xenguest.h
+++ b/tools/include/xenguest.h
@@ -223,6 +223,10 @@ struct xc_dom_image {
/* If unset disables the setup of the IOREQ pages. */
bool device_model;
+#if defined(__i386__) || defined(__x86_64__)
+ bool preenable_x2apic; /* 1 makes x2APIC enabled initially, 0 keeps default Xen behavior */
+#endif
+
/* BIOS/Firmware passed to HVMLOADER */
struct xc_hvm_firmware_module system_firmware_module;
diff --git a/tools/libs/guest/xg_dom_x86.c b/tools/libs/guest/xg_dom_x86.c
index a82b481a12..43ada5a6ac 100644
--- a/tools/libs/guest/xg_dom_x86.c
+++ b/tools/libs/guest/xg_dom_x86.c
@@ -58,6 +58,9 @@
#define MTRR_TYPE_WRBACK 6
#define MTRR_DEF_TYPE_ENABLE (1u << 11)
+#define APIC_BASE_EXTD (1UL << 10)
+#define APIC_BASE_ENABLE (1UL << 11)
+
#define SPECIALPAGE_PAGING 0
#define SPECIALPAGE_ACCESS 1
#define SPECIALPAGE_SHARING 2
@@ -1131,6 +1134,45 @@ static int vcpu_hvm(struct xc_dom_image *dom)
}
}
+ if ( dom->preenable_x2apic )
+ {
+ struct {
+ struct hvm_save_descriptor header_d;
+ HVM_SAVE_TYPE(HEADER) header;
+ struct hvm_save_descriptor lapic_d;
+ HVM_SAVE_TYPE(LAPIC) lapic;
+ struct hvm_save_descriptor end_d;
+ HVM_SAVE_TYPE(END) end;
+ } lapic = {
+ .header_d = bsp_ctx.header_d,
+ .header = bsp_ctx.header,
+ .lapic_d.typecode = HVM_SAVE_CODE(LAPIC),
+ .lapic_d.length = HVM_SAVE_LENGTH(LAPIC),
+ .end_d = bsp_ctx.end_d,
+ .end = bsp_ctx.end,
+ };
+ const HVM_SAVE_TYPE(LAPIC) *lapic_record =
+ hvm_get_save_record(full_ctx, HVM_SAVE_CODE(LAPIC), 0);
+
+ if ( !lapic_record )
+ {
+ xc_dom_panic(dom->xch, XC_INTERNAL_ERROR,
+ "%s: unable to get LAPIC save record", __func__);
+ goto out;
+ }
+
+ memcpy(&lapic.lapic, lapic_record, sizeof(lapic.lapic));
+
+ lapic.lapic.apic_base_msr |= APIC_BASE_ENABLE | APIC_BASE_EXTD;
+
+ rc = xc_domain_hvm_setcontext(dom->xch, dom->guest_domid,
+ (uint8_t *)&lapic, sizeof(lapic));
+
+ if ( rc != 0 )
+ xc_dom_panic(dom->xch, XC_INTERNAL_ERROR,
+ "%s: SETHVMCONTEXT failed (rc=%d)", __func__, rc);
+ }
+
/*
* Loading the BSP context should be done in the last call to setcontext,
* since each setcontext call will put all vCPUs down.
diff --git a/tools/libs/light/libxl_types.idl b/tools/libs/light/libxl_types.idl
index d64a573ff3..9fdf89d88b 100644
--- a/tools/libs/light/libxl_types.idl
+++ b/tools/libs/light/libxl_types.idl
@@ -738,6 +738,7 @@ libxl_domain_build_info = Struct("domain_build_info",[
("arm_sci", libxl_arm_sci),
])),
("arch_x86", Struct(None, [("msr_relaxed", libxl_defbool),
+ ("x2apic_preenable", libxl_defbool)
])),
# Alternate p2m is not bound to any architecture or guest type, as it is
# supported by x86 HVM and ARM support is planned.
diff --git a/tools/libs/light/libxl_x86.c b/tools/libs/light/libxl_x86.c
index 60d4e8661c..f9725f069a 100644
--- a/tools/libs/light/libxl_x86.c
+++ b/tools/libs/light/libxl_x86.c
@@ -555,6 +555,9 @@ int libxl__arch_domain_init_hw_description(libxl__gc *gc,
libxl__domain_build_state *state,
struct xc_dom_image *dom)
{
+ if (libxl_defbool_val(d_config->b_info.arch_x86.x2apic_preenable))
+ dom->preenable_x2apic = true;
+
return 0;
}
@@ -818,6 +821,7 @@ int libxl__arch_domain_build_info_setdefault(libxl__gc *gc,
{
libxl_defbool_setdefault(&b_info->acpi, true);
libxl_defbool_setdefault(&b_info->arch_x86.msr_relaxed, false);
+ libxl_defbool_setdefault(&b_info->arch_x86.x2apic_preenable, false);
libxl_defbool_setdefault(&b_info->trap_unmapped_accesses, false);
if (b_info->type == LIBXL_DOMAIN_TYPE_HVM) {
diff --git a/tools/xl/xl_parse.c b/tools/xl/xl_parse.c
index af86d3186d..92bf9d2ad5 100644
--- a/tools/xl/xl_parse.c
+++ b/tools/xl/xl_parse.c
@@ -3040,6 +3040,17 @@ skip_usbdev:
"WARNING: msr_relaxed will be removed in future versions.\n"
"If it fixes an issue you are having please report to "
"xen-devel@lists.xenproject.org.\n");
+
+ if (!xlu_cfg_get_string(config, "x2apic_mode", &buf, 1)) {
+ if (!strcmp(buf, "pre_enable"))
+ libxl_defbool_set(&b_info->arch_x86.x2apic_preenable, true);
+ else if (!strcmp(buf, "default"))
+ libxl_defbool_set(&b_info->arch_x86.x2apic_preenable, false);
+ else {
+ fprintf(stderr, "Unknown x2apic mode \"%s\" specified\n", buf);
+ exit(EXIT_FAILURE);
+ }
+ }
xlu_cfg_get_defbool(config, "vpmu", &b_info->vpmu, 0);
--
2.51.2
--
Teddy Astie | Vates XCP-ng Developer
XCP-ng & Xen Orchestra - Vates solutions
web: https://vates.techOn Wed Nov 12, 2025 at 3:51 PM CET, Teddy Astie wrote: > Introduce a new option to start the BSP vCPU in x2APIC mode instead > of xAPIC mode. Expose this in xl through a new "x2apic_mode" option. > > Signed-off-by: Teddy Astie <teddy.astie@vates.tech> > --- > Cc: Andrew Cooper <andrew.cooper3@citrix.com> > Cc: Jan Beulich <jbeulich@suse.com> > Cc: Alejandro Vallejo <alejandro.garciavallejo@amd.com> > Cc: Grygorii Strashko <grygorii_strashko@epam.com> > > Later on, we could consider with this option to use x2APIC ACPI > tables instead of xAPIC ones. > > There is also some room into introducing a new Kconfig option to > only support x2apic mode, which would change how the "Xen default" > would behave. > > changed in v2: > - only pre-enable instead of forcing > - use domain builder to pre-enable instead of introducing a new domain creation flag Hmmm. For dom0less/Hyperlaunch and CPU hotplug it'd be beneficial to actually have it in the domain creation struct, I think, annoying as this might sound (because it's a circle back to what you had before) could we keep the misc_flag with the different meaning of "preenable x2APIC" rather than "force"? Then on each vCPU creation (or even hotplug) we could check whether they need to be preinitialised as x2APIC or not. Otherwise hotplug needs different treatment. On the plus side, Hyperlaunch would merely have a new trivial binding rather than an ad-hoc solution. Cheers, Alejandro
Le 12/11/2025 à 15:51, Teddy Astie a écrit :
> Introduce a new option to start the BSP vCPU in x2APIC mode instead
> of xAPIC mode. Expose this in xl through a new "x2apic_mode" option.
>
> Signed-off-by: Teddy Astie <teddy.astie@vates.tech>
> ---
> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> Cc: Jan Beulich <jbeulich@suse.com>
> Cc: Alejandro Vallejo <alejandro.garciavallejo@amd.com>
> Cc: Grygorii Strashko <grygorii_strashko@epam.com>
>
> Later on, we could consider with this option to use x2APIC ACPI
> tables instead of xAPIC ones.
>
> There is also some room into introducing a new Kconfig option to
> only support x2apic mode, which would change how the "Xen default"
> would behave.
>
> changed in v2:
> - only pre-enable instead of forcing
> - use domain builder to pre-enable instead of introducing a new domain creation flag
>
> v1:
> - https://lore.kernel.org/xen-devel/d498a50f6187b362ac5da3c6a7a7c348f35dc4b3.1761761288.git.teddy.astie@vates.tech/
> ---
> docs/man/xl.cfg.5.pod.in | 16 ++++++++++++
> tools/include/libxl.h | 8 ++++++
> tools/include/xenguest.h | 4 +++
> tools/libs/guest/xg_dom_x86.c | 42 ++++++++++++++++++++++++++++++++
> tools/libs/light/libxl_types.idl | 1 +
> tools/libs/light/libxl_x86.c | 4 +++
> tools/xl/xl_parse.c | 11 +++++++++
> 7 files changed, 86 insertions(+)
>
> diff --git a/docs/man/xl.cfg.5.pod.in b/docs/man/xl.cfg.5.pod.in
> index ad1553c5e9..0f7a89fe92 100644
> --- a/docs/man/xl.cfg.5.pod.in
> +++ b/docs/man/xl.cfg.5.pod.in
> @@ -3198,6 +3198,22 @@ option.
>
> If using this option is necessary to fix an issue, please report a bug.
>
> +=item B<x2apic_mode="MODE">
> +
> +Sets the x2apic mode of the domain. The valid values are as follows:
> +
> +=over 4
> +
> +=item B<"default">
> +
> +Use default Xen LAPIC behavior.
> +
> +=item B<"pre_enable">
> +
> +Initially enable x2apic for the BSP of the domain.
> +
> +=back
> +
> =back
>
> =head1 SEE ALSO
> diff --git a/tools/include/libxl.h b/tools/include/libxl.h
> index bc35e412da..9850e8aa41 100644
> --- a/tools/include/libxl.h
> +++ b/tools/include/libxl.h
> @@ -1537,6 +1537,14 @@ void libxl_mac_copy(libxl_ctx *ctx, libxl_mac *dst, const libxl_mac *src);
> */
> #define LIBXL_HAVE_XEN_PLATFORM_PCI_BAR_UC
>
> +/*
> + * LIBXL_HAVE_X2APIC_PREENABLE
> + *
> + * libxl_domain_build_info contains a boolean 'arch_x86.x2apic_preenable' field
> + * to initially set the BSP LAPIC in x2APIC mode.
> + */
> +#define LIBXL_HAVE_X2APIC_PREENABLE
> +
> typedef char **libxl_string_list;
> void libxl_string_list_dispose(libxl_string_list *sl);
> int libxl_string_list_length(const libxl_string_list *sl);
> diff --git a/tools/include/xenguest.h b/tools/include/xenguest.h
> index c88958faa9..408a0c77e8 100644
> --- a/tools/include/xenguest.h
> +++ b/tools/include/xenguest.h
> @@ -223,6 +223,10 @@ struct xc_dom_image {
> /* If unset disables the setup of the IOREQ pages. */
> bool device_model;
>
> +#if defined(__i386__) || defined(__x86_64__)
> + bool preenable_x2apic; /* 1 makes x2APIC enabled initially, 0 keeps default Xen behavior */
> +#endif
> +
> /* BIOS/Firmware passed to HVMLOADER */
> struct xc_hvm_firmware_module system_firmware_module;
>
> diff --git a/tools/libs/guest/xg_dom_x86.c b/tools/libs/guest/xg_dom_x86.c
> index a82b481a12..43ada5a6ac 100644
> --- a/tools/libs/guest/xg_dom_x86.c
> +++ b/tools/libs/guest/xg_dom_x86.c
> @@ -58,6 +58,9 @@
> #define MTRR_TYPE_WRBACK 6
> #define MTRR_DEF_TYPE_ENABLE (1u << 11)
>
> +#define APIC_BASE_EXTD (1UL << 10)
> +#define APIC_BASE_ENABLE (1UL << 11)
> +
> #define SPECIALPAGE_PAGING 0
> #define SPECIALPAGE_ACCESS 1
> #define SPECIALPAGE_SHARING 2
> @@ -1131,6 +1134,45 @@ static int vcpu_hvm(struct xc_dom_image *dom)
> }
> }
>
> + if ( dom->preenable_x2apic )
> + {
> + struct {
> + struct hvm_save_descriptor header_d;
> + HVM_SAVE_TYPE(HEADER) header;
> + struct hvm_save_descriptor lapic_d;
> + HVM_SAVE_TYPE(LAPIC) lapic;
> + struct hvm_save_descriptor end_d;
> + HVM_SAVE_TYPE(END) end;
> + } lapic = {
> + .header_d = bsp_ctx.header_d,
> + .header = bsp_ctx.header,
> + .lapic_d.typecode = HVM_SAVE_CODE(LAPIC),
> + .lapic_d.length = HVM_SAVE_LENGTH(LAPIC),
> + .end_d = bsp_ctx.end_d,
> + .end = bsp_ctx.end,
> + };
> + const HVM_SAVE_TYPE(LAPIC) *lapic_record =
> + hvm_get_save_record(full_ctx, HVM_SAVE_CODE(LAPIC), 0);
> +
> + if ( !lapic_record )
> + {
> + xc_dom_panic(dom->xch, XC_INTERNAL_ERROR,
> + "%s: unable to get LAPIC save record", __func__);
> + goto out;
> + }
> +
> + memcpy(&lapic.lapic, lapic_record, sizeof(lapic.lapic));
> +
> + lapic.lapic.apic_base_msr |= APIC_BASE_ENABLE | APIC_BASE_EXTD;
> +
> + rc = xc_domain_hvm_setcontext(dom->xch, dom->guest_domid,
> + (uint8_t *)&lapic, sizeof(lapic));
> +
> + if ( rc != 0 )
> + xc_dom_panic(dom->xch, XC_INTERNAL_ERROR,
> + "%s: SETHVMCONTEXT failed (rc=%d)", __func__, rc);
> + }
> +
> /*
> * Loading the BSP context should be done in the last call to setcontext,
> * since each setcontext call will put all vCPUs down.
> diff --git a/tools/libs/light/libxl_types.idl b/tools/libs/light/libxl_types.idl
> index d64a573ff3..9fdf89d88b 100644
> --- a/tools/libs/light/libxl_types.idl
> +++ b/tools/libs/light/libxl_types.idl
> @@ -738,6 +738,7 @@ libxl_domain_build_info = Struct("domain_build_info",[
> ("arm_sci", libxl_arm_sci),
> ])),
> ("arch_x86", Struct(None, [("msr_relaxed", libxl_defbool),
> + ("x2apic_preenable", libxl_defbool)
> ])),
> # Alternate p2m is not bound to any architecture or guest type, as it is
> # supported by x86 HVM and ARM support is planned.
> diff --git a/tools/libs/light/libxl_x86.c b/tools/libs/light/libxl_x86.c
> index 60d4e8661c..f9725f069a 100644
> --- a/tools/libs/light/libxl_x86.c
> +++ b/tools/libs/light/libxl_x86.c
> @@ -555,6 +555,9 @@ int libxl__arch_domain_init_hw_description(libxl__gc *gc,
> libxl__domain_build_state *state,
> struct xc_dom_image *dom)
> {
> + if (libxl_defbool_val(d_config->b_info.arch_x86.x2apic_preenable))
> + dom->preenable_x2apic = true;
> +
> return 0;
> }
>
> @@ -818,6 +821,7 @@ int libxl__arch_domain_build_info_setdefault(libxl__gc *gc,
> {
> libxl_defbool_setdefault(&b_info->acpi, true);
> libxl_defbool_setdefault(&b_info->arch_x86.msr_relaxed, false);
> + libxl_defbool_setdefault(&b_info->arch_x86.x2apic_preenable, false);
> libxl_defbool_setdefault(&b_info->trap_unmapped_accesses, false);
>
> if (b_info->type == LIBXL_DOMAIN_TYPE_HVM) {
> diff --git a/tools/xl/xl_parse.c b/tools/xl/xl_parse.c
> index af86d3186d..92bf9d2ad5 100644
> --- a/tools/xl/xl_parse.c
> +++ b/tools/xl/xl_parse.c
> @@ -3040,6 +3040,17 @@ skip_usbdev:
> "WARNING: msr_relaxed will be removed in future versions.\n"
> "If it fixes an issue you are having please report to "
> "xen-devel@lists.xenproject.org.\n");
> +
> + if (!xlu_cfg_get_string(config, "x2apic_mode", &buf, 1)) {
> + if (!strcmp(buf, "pre_enable"))
> + libxl_defbool_set(&b_info->arch_x86.x2apic_preenable, true);
> + else if (!strcmp(buf, "default"))
> + libxl_defbool_set(&b_info->arch_x86.x2apic_preenable, false);
> + else {
> + fprintf(stderr, "Unknown x2apic mode \"%s\" specified\n", buf);
> + exit(EXIT_FAILURE);
> + }
> + }
>
> xlu_cfg_get_defbool(config, "vpmu", &b_info->vpmu, 0);
>
I haven't received a feedback on this. Aside the eventual future use of
x2APIC ACPI tables, are there objections for this patch ?
--
Teddy Astie | Vates XCP-ng Developer
XCP-ng & Xen Orchestra - Vates solutions
web: https://vates.tech
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