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Subject[PATCH v4 22/22] x86/fpu/xstate: Introduce boot-parameters to control state component support
Date
"xstate.disable=0x60000" will disable AMX on a system that has AMX compiled
into XFEATURE_MASK_USER_ENABLED.

"xstate.enable=0x60000" will enable AMX on a system that does NOT have AMX
compiled into XFEATURE_MASK_USER_ENABLED (assuming the kernel is new enough
to support this feature).

Rename XFEATURE_MASK_USER_SUPPORTED to XFEATURE_MASK_USER_ENABLED to be
aligned with the new parameters.

While this cmdline is currently enabled only for AMX, it is intended to be
easily enabled to be useful for future XSAVE-enabled features.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Reviewed-by: Len Brown <len.brown@intel.com>
Cc: x86@kernel.org
Cc: linux-doc@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
Changes from v3:
* Fixed a few typos. (Randy Dunlap)

Changes from v2:
* Changed the kernel tainted when any unknown state is enabled. (Andy
Lutomirski)
* Simplified the cmdline handling.
* Edited the changelog.

Changes from v1:
* Renamed the user state mask define (Andy Lutomirski and Dave Hansen)
* Changed the error message (Dave Hansen)
* Fixed xfeatures_mask_user()
* Rebased the upstream kernel (5.10) -- revived the param parse function
---
.../admin-guide/kernel-parameters.txt | 15 +++++
arch/x86/include/asm/fpu/types.h | 6 ++
arch/x86/include/asm/fpu/xstate.h | 24 +++----
arch/x86/kernel/fpu/init.c | 65 +++++++++++++++++--
4 files changed, 93 insertions(+), 17 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a10b545c2070..ec79f63979a4 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6014,6 +6014,21 @@
which allow the hypervisor to 'idle' the guest on lock
contention.

+ xstate.enable= [X86-64]
+ xstate.disable= [X86-64]
+ The kernel is compiled with a default xstate bitmask --
+ enabling it to use the XSAVE hardware to efficiently
+ save and restore thread states on context switch.
+ xstate.enable allows adding to that default mask at
+ boot-time without recompiling the kernel just to support
+ the new thread state. (Note that the kernel will ignore
+ any bits in the mask that do not correspond to features
+ that are actually available in CPUID.) xstate.disable
+ allows clearing bits in the default mask, forcing the
+ kernel to forget that it supports the specified thread
+ state. When a bit set for both, the kernel takes
+ xstate.disable as a priority.
+
xirc2ps_cs= [NET,PCMCIA]
Format:
<irq>,<irq_mask>,<io>,<full_duplex>,<do_sound>,<lockup_hack>[,<irq2>[,<irq3>[,<irq4>]]]
diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
index 2f297aa85d8f..967d38cc7eb1 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -149,6 +149,12 @@ enum xfeature {
#define XFEATURE_MASK_XTILE (XFEATURE_MASK_XTILE_DATA \
| XFEATURE_MASK_XTILE_CFG)

+#define XFEATURE_REGION_MASK(max_bit, min_bit) \
+ ((BIT_ULL((max_bit) - (min_bit) + 1) - 1) << (min_bit))
+
+#define XFEATURE_MASK_CONFIGURABLE \
+ XFEATURE_REGION_MASK(XFEATURE_XTILE_DATA, XFEATURE_XTILE_CFG)
+
#define FIRST_EXTENDED_XFEATURE XFEATURE_YMM

struct reg_128_bit {
diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
index 9e5c28f3beaa..1e64afea9f68 100644
--- a/arch/x86/include/asm/fpu/xstate.h
+++ b/arch/x86/include/asm/fpu/xstate.h
@@ -25,17 +25,17 @@

#define XSAVE_ALIGNMENT 64

-/* All currently supported user features */
-#define XFEATURE_MASK_USER_SUPPORTED (XFEATURE_MASK_FP | \
- XFEATURE_MASK_SSE | \
- XFEATURE_MASK_YMM | \
- XFEATURE_MASK_OPMASK | \
- XFEATURE_MASK_ZMM_Hi256 | \
- XFEATURE_MASK_Hi16_ZMM | \
- XFEATURE_MASK_PKRU | \
- XFEATURE_MASK_BNDREGS | \
- XFEATURE_MASK_BNDCSR | \
- XFEATURE_MASK_XTILE)
+/* All currently enabled user features */
+#define XFEATURE_MASK_USER_ENABLED (XFEATURE_MASK_FP | \
+ XFEATURE_MASK_SSE | \
+ XFEATURE_MASK_YMM | \
+ XFEATURE_MASK_OPMASK | \
+ XFEATURE_MASK_ZMM_Hi256 | \
+ XFEATURE_MASK_Hi16_ZMM | \
+ XFEATURE_MASK_PKRU | \
+ XFEATURE_MASK_BNDREGS | \
+ XFEATURE_MASK_BNDCSR | \
+ XFEATURE_MASK_XTILE)

/* All currently supported supervisor features */
#define XFEATURE_MASK_SUPERVISOR_SUPPORTED (XFEATURE_MASK_PASID)
@@ -87,7 +87,7 @@ static inline u64 xfeatures_mask_supervisor(void)

static inline u64 xfeatures_mask_user(void)
{
- return xfeatures_mask_all & XFEATURE_MASK_USER_SUPPORTED;
+ return xfeatures_mask_all & ~(XFEATURE_MASK_SUPERVISOR_ALL);
}

static inline u64 xfeatures_mask_supervisor_dynamic(void)
diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
index 046889f31037..0166d3eb9916 100644
--- a/arch/x86/kernel/fpu/init.c
+++ b/arch/x86/kernel/fpu/init.c
@@ -5,6 +5,7 @@
#include <asm/fpu/internal.h>
#include <asm/tlbflush.h>
#include <asm/setup.h>
+#include <asm/cmdline.h>

#include <linux/sched.h>
#include <linux/sched/task.h>
@@ -215,14 +216,45 @@ static void __init fpu__init_system_xstate_size_legacy(void)
/*
* Find supported xfeatures based on cpu features and command-line input.
* This must be called after fpu__init_parse_early_param() is called and
- * xfeatures_mask is enumerated.
+ * xfeatures_mask_all is enumerated.
*/
+
+static u64 xstate_enable;
+static u64 xstate_disable;
+
u64 __init fpu__get_supported_xfeatures_mask(void)
{
- u64 mask = XFEATURE_MASK_USER_SUPPORTED | XFEATURE_MASK_SUPERVISOR_SUPPORTED;
-
- if (!IS_ENABLED(CONFIG_X86_64))
- mask &= ~(XFEATURE_MASK_XTILE);
+ u64 mask = XFEATURE_MASK_USER_ENABLED | XFEATURE_MASK_SUPERVISOR_SUPPORTED;
+
+ if (!IS_ENABLED(CONFIG_X86_64)) {
+ mask &= ~(XFEATURE_MASK_XTILE);
+ } else if (xstate_enable || xstate_disable) {
+ u64 custom = mask;
+ u64 unknown;
+
+ custom |= xstate_enable;
+ custom &= ~xstate_disable;
+
+ unknown = custom & ~mask;
+ if (unknown) {
+ /*
+ * User should fully understand the result of using undocumented
+ * xstate component.
+ */
+ add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
+ pr_warn("x86/fpu: Attempt to enable unknown xstate features 0x%llx\n",
+ unknown);
+ WARN_ON_FPU(1);
+ }
+
+ if ((custom & XFEATURE_MASK_XTILE) != XFEATURE_MASK_XTILE) {
+ pr_warn("x86/fpu: Error in xstate.disable. Additionally disabling 0x%x components.\n",
+ XFEATURE_MASK_XTILE);
+ custom &= ~(XFEATURE_MASK_XTILE);
+ }
+
+ mask = custom;
+ }

return mask;
}
@@ -236,12 +268,35 @@ static void __init fpu__init_system_ctx_switch(void)
on_boot_cpu = 0;
}

+/*
+ * Longest parameter of 'xstate.enable=' is 22 octal number characters with '0' prefix and
+ * an extra '\0' for termination.
+ */
+#define MAX_XSTATE_MASK_CHARS 24
+/*
+ * We parse xstate parameters early because fpu__init_system() is executed before
+ * parse_early_param().
+ */
+static void __init fpu__init_parse_early_param(void)
+{
+ char arg[MAX_XSTATE_MASK_CHARS];
+
+ if (cmdline_find_option(boot_command_line, "xstate.enable", arg, sizeof(arg)) &&
+ !kstrtoull(arg, 0, &xstate_enable))
+ xstate_enable &= XFEATURE_MASK_CONFIGURABLE;
+
+ if (cmdline_find_option(boot_command_line, "xstate.disable", arg, sizeof(arg)) &&
+ !kstrtoull(arg, 0, &xstate_disable))
+ xstate_disable &= XFEATURE_MASK_CONFIGURABLE;
+}
+
/*
* Called on the boot CPU once per system bootup, to set up the initial
* FPU state that is later cloned into all processes:
*/
void __init fpu__init_system(struct cpuinfo_x86 *c)
{
+ fpu__init_parse_early_param();
fpu__init_system_early_generic(c);

/*
--
2.17.1
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