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Subject[PATCH v4 14/22] x86/fpu/xstate: Expand the xstate buffer on the first use of dynamic user state
Date
Intel's Extended Feature Disable (XFD) feature is an extension of the XSAVE
architecture. XFD allows the kernel to enable a feature state in XCR0 and
to receive a #NM trap when a task uses instructions accessing that state.
In this way, Linux can defer allocating the large XSAVE buffer until tasks
need it.

XFD introduces two MSRs: IA32_XFD to enable/disable the feature and
IA32_XFD_ERR to assist the #NM trap handler. Both use the same
state-component bitmap format, used by XCR0.

Use this hardware capability to find the right time to expand the xstate
buffer. Introduce two sets of helper functions for that:

1. The first set is primarily for interacting with the XFD hardware:
xdisable_setbits()
xdisable_getbits()
xdisable_switch()

2. The second set is for managing the first-use status and handling #NM
trap:
xfirstuse_enabled()
xfirstuse_not_detected()

The #NM handler induces the xstate buffer expansion to save the first-used
states.

The XFD feature is enabled only for the compacted format. If the kernel
uses the standard format, the buffer has to be always enough for all the
states.

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-kernel@vger.kernel.org
---
Changes from v3:
* Removed 'no functional change' in the changelog. (Borislav Petkov)

Changes from v2:
* Changed to enable XFD only when the compacted format is used.
* Updated the changelog with task->fpu removed. (Borislav Petkov)

Changes from v1:
* Inlined the XFD-induced #NM handling code (Andy Lutomirski)
---
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/include/asm/fpu/internal.h | 51 ++++++++++++++++++++++++++++-
arch/x86/include/asm/msr-index.h | 2 ++
arch/x86/kernel/cpu/cpuid-deps.c | 1 +
arch/x86/kernel/fpu/xstate.c | 37 +++++++++++++++++++--
arch/x86/kernel/process.c | 5 +++
arch/x86/kernel/process_32.c | 2 +-
arch/x86/kernel/process_64.c | 2 +-
arch/x86/kernel/traps.c | 40 ++++++++++++++++++++++
9 files changed, 135 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 84b887825f12..3170ab367cf2 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -277,6 +277,7 @@
#define X86_FEATURE_XSAVEC (10*32+ 1) /* XSAVEC instruction */
#define X86_FEATURE_XGETBV1 (10*32+ 2) /* XGETBV with ECX = 1 instruction */
#define X86_FEATURE_XSAVES (10*32+ 3) /* XSAVES/XRSTORS instructions */
+#define X86_FEATURE_XFD (10*32+ 4) /* eXtended Feature Disabling */

/*
* Extended auxiliary flags: Linux defined - for features scattered in various
diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index f964f3efc92e..c467312d38d8 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -557,11 +557,58 @@ static inline void switch_fpu_prepare(struct fpu *old_fpu, int cpu)
* Misc helper functions:
*/

+/* The first-use detection helpers: */
+
+static inline void xdisable_setbits(u64 value)
+{
+ wrmsrl_safe(MSR_IA32_XFD, value);
+}
+
+static inline u64 xdisable_getbits(void)
+{
+ u64 value;
+
+ rdmsrl_safe(MSR_IA32_XFD, &value);
+ return value;
+}
+
+static inline u64 xfirstuse_enabled(void)
+{
+ /* All the dynamic user components are first-use enabled. */
+ return xfeatures_mask_user_dynamic;
+}
+
+/*
+ * Convert fpu->state_mask to the xdisable configuration to be written to
+ * MSR IA32_XFD. So, xdisable_setbits() only uses this outcome.
+ */
+static inline u64 xfirstuse_not_detected(struct fpu *fpu)
+{
+ u64 firstuse_bv = (fpu->state_mask & xfirstuse_enabled());
+
+ /*
+ * If first-use is not detected, set the bit. If the detection is
+ * not enabled, the bit is always zero in firstuse_bv. So, make
+ * following conversion:
+ */
+ return (xfirstuse_enabled() ^ firstuse_bv);
+}
+
+/* Update MSR IA32_XFD with xfirstuse_not_detected() if needed. */
+static inline void xdisable_switch(struct fpu *prev, struct fpu *next)
+{
+ if (!static_cpu_has(X86_FEATURE_XFD) || !xfirstuse_enabled())
+ return;
+
+ if (unlikely(prev->state_mask != next->state_mask))
+ xdisable_setbits(xfirstuse_not_detected(next));
+}
+
/*
* Load PKRU from the FPU context if available. Delay loading of the
* complete FPU state until the return to userland.
*/
-static inline void switch_fpu_finish(struct fpu *new_fpu)
+static inline void switch_fpu_finish(struct fpu *old_fpu, struct fpu *new_fpu)
{
u32 pkru_val = init_pkru_value;
struct pkru_state *pk;
@@ -571,6 +618,8 @@ static inline void switch_fpu_finish(struct fpu *new_fpu)

set_thread_flag(TIF_NEED_FPU_LOAD);

+ xdisable_switch(old_fpu, new_fpu);
+
if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
return;

diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 546d6ecf0a35..eb65e836c2d1 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -622,6 +622,8 @@
#define MSR_IA32_BNDCFGS_RSVD 0x00000ffc

#define MSR_IA32_XSS 0x00000da0
+#define MSR_IA32_XFD 0x000001c4
+#define MSR_IA32_XFD_ERR 0x000001c5

#define MSR_IA32_APICBASE 0x0000001b
#define MSR_IA32_APICBASE_BSP (1<<8)
diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
index 42af31b64c2c..4423046c2d74 100644
--- a/arch/x86/kernel/cpu/cpuid-deps.c
+++ b/arch/x86/kernel/cpu/cpuid-deps.c
@@ -72,6 +72,7 @@ static const struct cpuid_dep cpuid_deps[] = {
{ X86_FEATURE_AVX512_FP16, X86_FEATURE_AVX512BW },
{ X86_FEATURE_ENQCMD, X86_FEATURE_XSAVES },
{ X86_FEATURE_PER_THREAD_MBA, X86_FEATURE_MBA },
+ { X86_FEATURE_XFD, X86_FEATURE_XSAVES },
{}
};

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index c57877df797d..b69913ae30ed 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -175,6 +175,21 @@ static bool xfeature_is_supervisor(int xfeature_nr)
return ecx & 1;
}

+static bool xfeature_disable_supported(int xfeature_nr)
+{
+ u32 eax, ebx, ecx, edx;
+
+ if (!boot_cpu_has(X86_FEATURE_XFD))
+ return false;
+
+ /*
+ * If state component 'i' supports xfeature disable (first-use
+ * detection), ECX[2] return 1; otherwise, 0.
+ */
+ cpuid_count(XSTATE_CPUID, xfeature_nr, &eax, &ebx, &ecx, &edx);
+ return ecx & 4;
+}
+
/**
* get_xstate_comp_offset() - Find the feature's offset in the compacted format
* @mask: This bitmap tells which components reserved in the format.
@@ -366,6 +381,9 @@ void fpu__init_cpu_xstate(void)
wrmsrl(MSR_IA32_XSS, xfeatures_mask_supervisor() |
xfeatures_mask_supervisor_dynamic());
}
+
+ if (boot_cpu_has(X86_FEATURE_XFD))
+ xdisable_setbits(xfirstuse_enabled());
}

static bool xfeature_enabled(enum xfeature xfeature)
@@ -565,8 +583,9 @@ static void __init print_xstate_offset_size(void)
for (i = FIRST_EXTENDED_XFEATURE; i < XFEATURE_MAX; i++) {
if (!xfeature_enabled(i))
continue;
- pr_info("x86/fpu: xstate_offset[%d]: %4d, xstate_sizes[%d]: %4d\n",
- i, xstate_comp_offsets[i], i, xstate_sizes[i]);
+ pr_info("x86/fpu: xstate_offset[%d]: %4d, xstate_sizes[%d]: %4d (%s)\n",
+ i, xstate_comp_offsets[i], i, xstate_sizes[i],
+ (xfeatures_mask_user_dynamic & BIT_ULL(i)) ? "on-demand" : "default");
}
}

@@ -999,9 +1018,18 @@ void __init fpu__init_system_xstate(void)
}

xfeatures_mask_all &= fpu__get_supported_xfeatures_mask();
- /* Do not support the dynamically allocated buffer yet. */
xfeatures_mask_user_dynamic = 0;

+ for (i = FIRST_EXTENDED_XFEATURE; i < XFEATURE_MAX; i++) {
+ u64 feature_mask = BIT_ULL(i);
+
+ if (!(xfeatures_mask_user() & feature_mask))
+ continue;
+
+ if (xfeature_disable_supported(i))
+ xfeatures_mask_user_dynamic |= feature_mask;
+ }
+
/* Enable xstate instructions to be able to continue with initialization: */
fpu__init_cpu_xstate();
err = init_xstate_size();
@@ -1053,6 +1081,9 @@ void fpu__resume_cpu(void)
wrmsrl(MSR_IA32_XSS, xfeatures_mask_supervisor() |
xfeatures_mask_supervisor_dynamic());
}
+
+ if (boot_cpu_has(X86_FEATURE_XFD))
+ xdisable_setbits(xfirstuse_not_detected(&current->thread.fpu));
}

/*
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index c41116543a82..697d554a242d 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -103,6 +103,11 @@ void arch_thread_struct_whitelist(unsigned long *offset, unsigned long *size)
*size = get_xstate_config(XSTATE_MIN_SIZE);
}

+void arch_release_task_struct(struct task_struct *tsk)
+{
+ free_xstate_buffer(&tsk->thread.fpu);
+}
+
/*
* Free thread data structures etc..
*/
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 4f2f54e1281c..7bd5d08eeb41 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -213,7 +213,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)

this_cpu_write(current_task, next_p);

- switch_fpu_finish(next_fpu);
+ switch_fpu_finish(prev_fpu, next_fpu);

/* Load the Intel cache allocation PQR MSR. */
resctrl_sched_in();
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index ad582f9ac5a6..6fb44c4ceeee 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -594,7 +594,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
this_cpu_write(current_task, next_p);
this_cpu_write(cpu_current_top_of_stack, task_top_of_stack(next_p));

- switch_fpu_finish(next_fpu);
+ switch_fpu_finish(prev_fpu, next_fpu);

/* Reload sp0. */
update_task_stack(next_p);
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 7f5aec758f0e..821a7f408ad4 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -1106,10 +1106,50 @@ DEFINE_IDTENTRY(exc_spurious_interrupt_bug)
*/
}

+static __always_inline bool handle_xfirstuse_event(struct fpu *fpu)
+{
+ bool handled = false;
+ u64 event_mask;
+
+ /* Check whether the first-use detection is running. */
+ if (!static_cpu_has(X86_FEATURE_XFD) || !xfirstuse_enabled())
+ return handled;
+
+ rdmsrl_safe(MSR_IA32_XFD_ERR, &event_mask);
+
+ /* The trap event should happen to one of first-use enabled features */
+ WARN_ON(!(event_mask & xfirstuse_enabled()));
+
+ /* If IA32_XFD_ERR is empty, the current trap has nothing to do with. */
+ if (!event_mask)
+ return handled;
+
+ /*
+ * The first-use event is presumed to be from userspace, so it should have
+ * nothing to do with interrupt context.
+ */
+ if (WARN_ON(in_interrupt()))
+ return handled;
+
+ if (alloc_xstate_buffer(fpu, event_mask))
+ return handled;
+
+ xdisable_setbits(xfirstuse_not_detected(fpu));
+
+ /* Clear the trap record. */
+ wrmsrl_safe(MSR_IA32_XFD_ERR, 0);
+ handled = true;
+
+ return handled;
+}
+
DEFINE_IDTENTRY(exc_device_not_available)
{
unsigned long cr0 = read_cr0();

+ if (handle_xfirstuse_event(&current->thread.fpu))
+ return;
+
#ifdef CONFIG_MATH_EMULATION
if (!boot_cpu_has(X86_FEATURE_FPU) && (cr0 & X86_CR0_EM)) {
struct math_emu_info info = { };
--
2.17.1
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