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Subject[PATCH v10 09/28] x86/fpu/xstate: Introduce helpers to manage the XSTATE buffer dynamically
Date
The static XSTATE per-task buffer contains the extended register states --
but it is not expandable at runtime. Introduce runtime methods and a new
fpu struct field to support the expansion.

fpu->state_mask indicates which state components are to be saved in the
XSTATE buffer.

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

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

Also, use the new field and helper to initialize the buffer.

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 v9:
* Rename and simplify helpers. (Borislav Petkov)
* Add and fix the code comment and the variable name. (Borislav Petkov)
* Use cpu_feature_enabled() instead of boot_cpu_has(). (Borislav Petkov)
* Use fpu->state_mask to ensure states to be written in
copy_uabi_to_xstate() -- moved from Patch11. (Borislav Petkov)

Changes from v5:
* Added to ensure XSAVES format with current in fpu_reset_fpstate() for new
base code.

Changes from v3:
* Updated code comments. (Borislav Petkov)
* Used vzalloc() instead of vmalloc() with memset(). (Borislav Petkov)
* Removed the max size check for >64KB. (Borislav Petkov)
* Removed the allocation size check in the helper. (Borislav Petkov)
* Switched the function description in the kernel-doc style.
* Used them for buffer initialization -- moved from the next patch.

Changes from v2:
* Updated the changelog with task->fpu removed. (Borislav 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 | 7 ++
arch/x86/include/asm/fpu/xstate.h | 3 +
arch/x86/kernel/fpu/core.c | 19 +++--
arch/x86/kernel/fpu/xstate.c | 123 ++++++++++++++++++++++++++++++
4 files changed, 145 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h
index ad5cbf922e30..0cc9f6c5a10c 100644
--- a/arch/x86/include/asm/fpu/types.h
+++ b/arch/x86/include/asm/fpu/types.h
@@ -336,6 +336,13 @@ struct fpu {
*/
unsigned long avx512_timestamp;

+ /*
+ * @state_mask:
+ *
+ * The bitmap represents state components to be saved in ->state.
+ */
+ u64 state_mask;
+
/*
* @state:
*
diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
index c4a0914b7717..cbe19e05b5c9 100644
--- a/arch/x86/include/asm/fpu/xstate.h
+++ b/arch/x86/include/asm/fpu/xstate.h
@@ -153,7 +153,10 @@ struct fpu_xstate_buffer_config {

extern struct fpu_xstate_buffer_config fpu_buf_cfg;

+unsigned int calculate_xstate_buf_size_from_mask(u64 mask);
void *get_xsave_addr(struct fpu *fpu, int xfeature_nr);
+int realloc_xstate_buffer(struct fpu *fpu, u64 mask);
+void free_xstate_buffer(struct fpu *fpu);
int xfeature_size(int xfeature_nr);
int copy_uabi_from_kernel_to_xstate(struct fpu *fpu, const void *kbuf);
int copy_sigframe_from_user_to_xstate(struct fpu *fpu, const void __user *ubuf);
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 6b55b8c651f6..2941d03912db 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -236,9 +236,8 @@ void fpstate_init(struct fpu *fpu)

if (likely(fpu)) {
state = fpu->state;
- /* The dynamic user states are not prepared yet. */
- mask = xfeatures_mask_all & ~xfeatures_mask_user_dynamic;
- size = fpu_buf_cfg.min_size;
+ mask = fpu->state_mask;
+ size = calculate_xstate_buf_size_from_mask(fpu->state_mask);
} else {
state = &init_fpstate;
mask = xfeatures_mask_all;
@@ -274,14 +273,16 @@ int fpu_clone(struct task_struct *dst)
if (!cpu_feature_enabled(X86_FEATURE_FPU))
return 0;

+ /*
+ * The child does not inherit the dynamic states. Thus, use the
+ * buffer embedded in struct task_struct, which has the minimum
+ * size.
+ */
+ dst_fpu->state_mask = (xfeatures_mask_all & ~xfeatures_mask_user_dynamic);
dst_fpu->state = &dst_fpu->__default_state;
-
/*
* Don't let 'init optimized' areas of the XSAVE area
* leak into the child task:
- *
- * The child does not inherit the dynamic states. So,
- * the xstate buffer has the minimum size.
*/
memset(&dst_fpu->state->xsave, 0, fpu_buf_cfg.min_size);

@@ -380,6 +381,10 @@ static void fpu_reset_fpstate(void)
* flush_thread().
*/
memcpy(fpu->state, &init_fpstate, init_fpstate_copy_size());
+ /* Adjust the xstate buffer format for current. */
+ if (cpu_feature_enabled(X86_FEATURE_XSAVES))
+ fpstate_init_xstate(&fpu->state->xsave, fpu->state_mask);
+
set_thread_flag(TIF_NEED_FPU_LOAD);
fpregs_unlock();
}
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 4496750208a8..f56db3f77282 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
@@ -76,6 +78,12 @@ static unsigned int xstate_comp_offsets[XFEATURE_MAX] __ro_after_init =
{ [ 0 ... XFEATURE_MAX - 1] = -1};
static unsigned int xstate_supervisor_only_offsets[XFEATURE_MAX] __ro_after_init =
{ [ 0 ... XFEATURE_MAX - 1] = -1};
+/*
+ * True if the buffer of the corresponding XFEATURE is located on the next 64
+ * byte boundary. Otherwise, it follows the preceding component immediately.
+ */
+static bool xstate_64byte_aligned[XFEATURE_MAX] __ro_after_init =
+ { [ 0 ... XFEATURE_MAX - 1] = false};

struct fpu_xstate_buffer_config fpu_buf_cfg __ro_after_init;
EXPORT_SYMBOL_GPL(fpu_buf_cfg);
@@ -131,6 +139,58 @@ static bool xfeature_is_supervisor(int xfeature_nr)
return ecx & 1;
}

+/**
+ * calculate_xstate_buf_size_from_mask - Calculate an xstate buffer size
+ * @mask: A bitmap to tell which components to be saved in the buffer.
+ *
+ * Available once those arrays for the offset, size, and alignment info are
+ * set up, by setup_xstate_features().
+ *
+ * Returns: The buffer size
+ */
+unsigned int calculate_xstate_buf_size_from_mask(u64 mask)
+{
+ unsigned int size = FXSAVE_SIZE + XSAVE_HDR_SIZE;
+ int i, nr;
+
+ if (!mask)
+ return 0;
+
+ /*
+ * The minimum buffer size excludes the dynamic user state. When a
+ * task uses the state, the buffer can grow up to the max size.
+ */
+ if (mask == (xfeatures_mask_all & ~xfeatures_mask_user_dynamic))
+ return fpu_buf_cfg.min_size;
+ else if (mask == xfeatures_mask_all)
+ return fpu_buf_cfg.max_size;
+
+ nr = fls64(mask) - 1;
+ if (nr < FIRST_EXTENDED_XFEATURE)
+ return size;
+
+ /*
+ * Each state offset in the non-compacted format is fixed. Take the
+ * size from the last feature 'nr'.
+ */
+ if (!cpu_feature_enabled(X86_FEATURE_XSAVES))
+ return xstate_offsets[nr] + xstate_sizes[nr];
+
+ /*
+ * With the given mask, no relevant size is found so far. So,
+ * calculate it by summing up each state size.
+ */
+ for (i = FIRST_EXTENDED_XFEATURE; i <= nr; i++) {
+ if (!(mask & BIT_ULL(i)))
+ continue;
+
+ if (xstate_64byte_aligned[i])
+ size = ALIGN(size, 64);
+ size += xstate_sizes[i];
+ }
+ return size;
+}
+
/*
* Enable the extended processor state save/restore feature.
* Called once per CPU onlining.
@@ -202,6 +262,7 @@ static void __init setup_xstate_features(void)
continue;

xstate_offsets[i] = ebx;
+ xstate_64byte_aligned[i] = (ecx & 2) ? true : false;

/*
* In our xstate size checks, we assume that the highest-numbered
@@ -805,6 +866,12 @@ void __init fpu__init_system_xstate(void)
if (err)
goto out_disable;

+ /*
+ * Initially, the FPU buffer used is the static one, without
+ * dynamic 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:
@@ -995,6 +1062,60 @@ int arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
}
#endif /* ! CONFIG_ARCH_HAS_PKEYS */

+void free_xstate_buffer(struct fpu *fpu)
+{
+ vfree(fpu->state);
+}
+
+/**
+ * realloc_xstate_buffer - Re-alloc a buffer with the size calculated from
+ * @mask.
+ *
+ * @fpu: A struct fpu * pointer
+ * @mask: The bitmap tells which components to be reserved in the new
+ * buffer.
+ *
+ * It deals with enlarging the xstate buffer with dynamic states.
+ *
+ * Use vzalloc() simply here. If the task with a vzalloc()-allocated buffer
+ * tends to terminate quickly, vfree()-induced IPIs may be a concern.
+ * Caching may be helpful for this. But the task with large state is likely
+ * to live longer.
+ *
+ * Also, this method does not shrink or reclaim the buffer.
+ *
+ * Returns 0 on success, -ENOMEM on allocation error.
+ */
+int realloc_xstate_buffer(struct fpu *fpu, u64 mask)
+{
+ union fpregs_state *state;
+ u64 state_mask;
+
+ state_mask = fpu->state_mask | mask;
+ if ((state_mask & fpu->state_mask) == state_mask)
+ return 0;
+
+ state = vzalloc(calculate_xstate_buf_size_from_mask(state_mask));
+ if (!state)
+ return -ENOMEM;
+
+ /*
+ * As long as the register state is intact, save the xstate in the
+ * new buffer at the next context switch or ptrace's context
+ * injection.
+ */
+ if (cpu_feature_enabled(X86_FEATURE_XSAVES))
+ fpstate_init_xstate(&state->xsave, state_mask);
+
+ /* Free the old buffer */
+ if (fpu->state != &fpu->__default_state)
+ free_xstate_buffer(fpu);
+
+ fpu->state = state;
+ fpu->state_mask = state_mask;
+ return 0;
+}
+
static void copy_feature(bool from_xstate, struct membuf *to, void *xstate,
void *init_xstate, unsigned int size)
{
@@ -1147,6 +1268,8 @@ static int copy_uabi_to_xstate(struct fpu *fpu, const void *kbuf,
if (validate_user_xstate_header(&hdr))
return -EINVAL;

+ hdr.xfeatures &= fpu->state_mask;
+
/* Validate MXCSR when any of the related features is in use */
mask = XFEATURE_MASK_FP | XFEATURE_MASK_SSE | XFEATURE_MASK_YMM;
if (hdr.xfeatures & mask) {
--
2.17.1
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