[PATCH v3 07/21] x86/fpu/xstate: Introduce helpers to manage dynamic xstate buffers

From: Chang S. Bae
Date: Wed Dec 23 2020 - 11:03:11 EST

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The static per-task xstate buffer contains the extended register states --
but it is not expandable at runtime. Introduce runtime methods and new fpu
struct fields to support the expansion.

fpu->state_mask indicates the saved states per task and fpu->state_ptr
points to the dynamically allocated buffer.

alloc_xstate_buffer() uses vmalloc(). If use of this mechanism grows to
allocate buffers larger than 64KB, a more sophisticated allocation scheme
that includes purpose-built reclaim capability might be justified.

Introduce a new helper -- get_xstate_size() to calculate the buffer size.

No functional change until the kernel supports dynamic user states.

Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Reviewed-by: Len Brown <len.brown@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
Changes from v2:
* Updated the changelog with task->fpu removed. (Boris Petkov)
* Replaced 'area' with 'buffer' in the comments and the changelog.
* Updated the code comments.

Changes from v1:
* Removed unneeded interrupt masking (Andy Lutomirski)
* Added vmalloc() error tracing (Dave Hansen, PeterZ, and Andy Lutomirski)
---
arch/x86/include/asm/fpu/types.h | 29 ++++++--
arch/x86/include/asm/fpu/xstate.h | 3 +
arch/x86/include/asm/trace/fpu.h | 5 ++
arch/x86/kernel/fpu/core.c | 3 +
arch/x86/kernel/fpu/xstate.c | 115 ++++++++++++++++++++++++++++++
5 files changed, 150 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
index f5a38a5f3ae1..3fc6dbbe3ede 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -336,14 +336,33 @@ struct fpu {
*/
unsigned long avx512_timestamp;

+ /*
+ * @state_mask:
+ *
+ * The state component bitmap. It indicates the saved xstate in
+ * either @state or @state_ptr. The map value starts to be aligned
+ * with @state and then with @state_ptr once it is in use.
+ */
+ u64 state_mask;
+
+ /*
+ * @state_ptr:
+ *
+ * Copy of all extended register states, in a dynamically allocated
+ * buffer. When a task is using extended features, the register state
+ * is always the most current. This state copy is more recent than
+ * @state. If the task context-switches away, they get saved here,
+ * representing the xstate.
+ */
+ union fpregs_state *state_ptr;
+
/*
* @state:
*
- * In-memory copy of all FPU registers that we save/restore
- * over context switches. If the task is using the FPU then
- * the registers in the FPU are more recent than this state
- * copy. If the task context-switches away then they get
- * saved here and represent the FPU state.
+ * Copy of some extended register state. If a task uses a dynamically
+ * allocated buffer, @state_ptr, then it has a more recent state copy
+ * than this. This copy follows the same attributes as described for
+ * @state_ptr.
*/
union fpregs_state state;
/*
diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
index 6ce8350672c2..379e8f8b8440 100644
--- a/arch/x86/include/asm/fpu/xstate.h
+++ b/arch/x86/include/asm/fpu/xstate.h
@@ -103,6 +103,9 @@ extern void __init update_regset_xstate_info(unsigned int size,
u64 xstate_mask);

void *get_xsave_addr(struct fpu *fpu, int xfeature_nr);
+int alloc_xstate_buffer(struct fpu *fpu, u64 mask);
+void free_xstate_buffer(struct fpu *fpu);
+
const void *get_xsave_field_ptr(int xfeature_nr);
int using_compacted_format(void);
int xfeature_size(int xfeature_nr);
diff --git a/arch/x86/include/asm/trace/fpu.h b/arch/x86/include/asm/trace/fpu.h
index 879b77792f94..bf88b3333873 100644
--- a/arch/x86/include/asm/trace/fpu.h
+++ b/arch/x86/include/asm/trace/fpu.h
@@ -89,6 +89,11 @@ DEFINE_EVENT(x86_fpu, x86_fpu_xstate_check_failed,
TP_ARGS(fpu)
);

+DEFINE_EVENT(x86_fpu, x86_fpu_xstate_alloc_failed,
+ TP_PROTO(struct fpu *fpu),
+ TP_ARGS(fpu)
+);
+
#undef TRACE_INCLUDE_PATH
#define TRACE_INCLUDE_PATH asm/trace/
#undef TRACE_INCLUDE_FILE
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 1a428803e6b2..6dafed34be4f 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -235,6 +235,9 @@ int fpu__copy(struct task_struct *dst, struct task_struct *src)
*/
memset(&dst_fpu->state.xsave, 0, fpu_kernel_xstate_min_size);

+ dst_fpu->state_mask = xfeatures_mask_all & ~xfeatures_mask_user_dynamic;
+ dst_fpu->state_ptr = NULL;
+
/*
* If the FPU registers are not current just memcpy() the state.
* Otherwise save current FPU registers directly into the child's FPU
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 2012b17b1793..af4d7d9aa977 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -10,6 +10,7 @@
#include <linux/pkeys.h>
#include <linux/seq_file.h>
#include <linux/proc_fs.h>
+#include <linux/vmalloc.h>

#include <asm/fpu/api.h>
#include <asm/fpu/internal.h>
@@ -19,6 +20,7 @@

#include <asm/tlbflush.h>
#include <asm/cpufeature.h>
+#include <asm/trace/fpu.h>

/*
* Although we spell it out in here, the Processor Trace
@@ -71,6 +73,7 @@ static unsigned int xstate_offsets[XFEATURE_MAX] = { [ 0 ... XFEATURE_MAX - 1] =
static unsigned int xstate_sizes[XFEATURE_MAX] = { [ 0 ... XFEATURE_MAX - 1] = -1};
static unsigned int xstate_comp_offsets[XFEATURE_MAX] = { [ 0 ... XFEATURE_MAX - 1] = -1};
static unsigned int xstate_supervisor_only_offsets[XFEATURE_MAX] = { [ 0 ... XFEATURE_MAX - 1] = -1};
+static bool xstate_aligns[XFEATURE_MAX] = { [ 0 ... XFEATURE_MAX - 1] = false};

/*
* The XSAVE area of kernel can be in standard or compacted format;
@@ -130,6 +133,48 @@ static bool xfeature_is_supervisor(int xfeature_nr)
return ecx & 1;
}

+/*
+ * Available once those arrays for the offset, size, and alignment info are set up,
+ * by setup_xstate_features().
+ */
+static unsigned int get_xstate_size(u64 mask)
+{
+ unsigned int size;
+ u64 xmask;
+ int i, nr;
+
+ if (!mask)
+ return 0;
+ else if (mask == (xfeatures_mask_all & ~xfeatures_mask_user_dynamic))
+ return fpu_kernel_xstate_min_size;
+ else if (mask == xfeatures_mask_all)
+ return fpu_kernel_xstate_max_size;
+
+ nr = fls64(mask) - 1;
+
+ if (!using_compacted_format())
+ return xstate_offsets[nr] + xstate_sizes[nr];
+
+ xmask = BIT_ULL(nr + 1) - 1;
+
+ if (mask == (xmask & xfeatures_mask_all))
+ return xstate_comp_offsets[nr] + xstate_sizes[nr];
+
+ /*
+ * Calculate the size by summing up each state together, since no known
+ * size found with the xstate buffer format out of the given mask.
+ */
+ for (size = FXSAVE_SIZE + XSAVE_HDR_SIZE, i = FIRST_EXTENDED_XFEATURE; i <= nr; i++) {
+ if (!(mask & BIT_ULL(i)))
+ continue;
+
+ if (xstate_aligns[i])
+ size = ALIGN(size, 64);
+ size += xstate_sizes[i];
+ }
+ return size;
+}
+
/*
* When executing XSAVEOPT (or other optimized XSAVE instructions), if
* a processor implementation detects that an FPU state component is still
@@ -270,10 +315,12 @@ static void __init setup_xstate_features(void)
xstate_offsets[XFEATURE_FP] = 0;
xstate_sizes[XFEATURE_FP] = offsetof(struct fxregs_state,
xmm_space);
+ xstate_aligns[XFEATURE_FP] = true;

xstate_offsets[XFEATURE_SSE] = xstate_sizes[XFEATURE_FP];
xstate_sizes[XFEATURE_SSE] = sizeof_field(struct fxregs_state,
xmm_space);
+ xstate_aligns[XFEATURE_SSE] = true;

for (i = FIRST_EXTENDED_XFEATURE; i < XFEATURE_MAX; i++) {
if (!xfeature_enabled(i))
@@ -291,6 +338,7 @@ static void __init setup_xstate_features(void)
continue;

xstate_offsets[i] = ebx;
+ xstate_aligns[i] = (ecx & 2) ? true : false;

/*
* In our xstate size checks, we assume that the highest-numbered
@@ -755,6 +803,9 @@ static bool is_supported_xstate_size(unsigned int test_xstate_size)
return false;
}

+/* The watched threshold size of dynamically allocated xstate buffer */
+#define XSTATE_BUFFER_MAX_BYTES (64 * 1024)
+
static int __init init_xstate_size(void)
{
/* Recompute the context size for enabled features: */
@@ -779,6 +830,14 @@ static int __init init_xstate_size(void)
if (!is_supported_xstate_size(fpu_kernel_xstate_min_size))
return -EINVAL;

+ /*
+ * When allocating buffers larger than the threshold, a more sophisticated
+ * mechanism might be considerable.
+ */
+ if (fpu_kernel_xstate_max_size > XSTATE_BUFFER_MAX_BYTES)
+ pr_warn("x86/fpu: xstate buffer too large (%u > %u)\n",
+ fpu_kernel_xstate_max_size, XSTATE_BUFFER_MAX_BYTES);
+
/*
* User space is always in standard format.
*/
@@ -869,6 +928,9 @@ void __init fpu__init_system_xstate(void)
if (err)
goto out_disable;

+ /* Make sure init_task does not include the dynamic user states */
+ current->thread.fpu.state_mask = (xfeatures_mask_all & ~xfeatures_mask_user_dynamic);
+
/*
* Update info used for ptrace frames; use standard-format size and no
* supervisor xstates:
@@ -1089,6 +1151,59 @@ static inline bool xfeatures_mxcsr_quirk(u64 xfeatures)
return true;
}

+void free_xstate_buffer(struct fpu *fpu)
+{
+ vfree(fpu->state_ptr);
+}
+
+/*
+ * Allocate an xstate buffer with the size calculated based on 'mask'.
+ *
+ * The allocation mechanism does not shrink or reclaim the buffer.
+ */
+int alloc_xstate_buffer(struct fpu *fpu, u64 mask)
+{
+ union fpregs_state *state_ptr;
+ unsigned int oldsz, newsz;
+ u64 state_mask;
+
+ state_mask = fpu->state_mask | mask;
+
+ oldsz = get_xstate_size(fpu->state_mask);
+ newsz = get_xstate_size(state_mask);
+
+ if (oldsz >= newsz)
+ return 0;
+
+ if (newsz > fpu_kernel_xstate_max_size) {
+ pr_warn_once("x86/fpu: xstate buffer too large (%u > %u bytes)\n",
+ newsz, fpu_kernel_xstate_max_size);
+ XSTATE_WARN_ON(1);
+ return 0;
+ }
+
+ /* We need 64B aligned pointer, but vmalloc() returns a page-aligned address. */
+ state_ptr = vmalloc(newsz);
+ if (!state_ptr) {
+ trace_x86_fpu_xstate_alloc_failed(fpu);
+ return -ENOMEM;
+ }
+
+ memset(state_ptr, 0, newsz);
+ if (using_compacted_format())
+ fpstate_init_xstate(&state_ptr->xsave, state_mask);
+
+ /*
+ * As long as the register state is intact, save the xstate in the new buffer
+ * at the next context copy/switch or potentially ptrace-driven xstate writing.
+ */
+
+ vfree(fpu->state_ptr);
+ fpu->state_ptr = state_ptr;
+ fpu->state_mask = state_mask;
+ return 0;
+}
+
static void fill_gap(struct membuf *to, unsigned *last, unsigned offset)
{
if (*last >= offset)
--
2.17.1

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