linux-kernel - [RFC PATCH 14/22] x86/fpu/xstate: Inherit dynamic user state when used in the parent

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Date:   Thu,  1 Oct 2020 13:39:05 -0700
From:   "Chang S. Bae" <chang.seok.bae@...el.com>
To:     tglx@...utronix.de, mingo@...nel.org, bp@...e.de, luto@...nel.org,
        x86@...nel.org
Cc:     len.brown@...el.com, dave.hansen@...el.com, jing2.liu@...el.com,
        ravi.v.shankar@...el.com, linux-kernel@...r.kernel.org,
        chang.seok.bae@...el.com
Subject: [RFC PATCH 14/22] x86/fpu/xstate: Inherit dynamic user state when used in the parent

When a new task is created, the kernel copies all the states from the
parent. If the parent already has any dynamic user state in use, the new
task has to expand the XSAVE buffer to save them. Also, disable the
associated first-use fault.

No functional change until the kernel supports dynamic user states.

Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
Reviewed-by: Len Brown <len.brown@...el.com>
Cc: x86@...nel.org
Cc: linux-kernel@...r.kernel.org
---
 arch/x86/kernel/fpu/core.c | 28 ++++++++++++++++++++--------
 1 file changed, 20 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 2e07bfcd54b3..239c7798bc01 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -225,6 +225,7 @@ int fpu__copy(struct task_struct *dst, struct task_struct *src)
 {
 	struct fpu *dst_fpu = &dst->thread.fpu;
 	struct fpu *src_fpu = &src->thread.fpu;
+	unsigned int size;
 
 	dst_fpu->last_cpu = -1;
 
@@ -233,15 +234,26 @@ int fpu__copy(struct task_struct *dst, struct task_struct *src)
 
 	WARN_ON_FPU(src_fpu != &current->thread.fpu);
 
-	/*
-	 * Don't let 'init optimized' areas of the XSAVE area
-	 * leak into the child task:
-	 */
-	memset(&dst_fpu->state.xsave, 0, fpu_kernel_xstate_default_size);
-
-	dst_fpu->state_mask = xfeatures_mask_all & ~xfeatures_mask_user_dynamic;
 	dst_fpu->state_ptr = NULL;
 
+	/* Inherit the dynamic area if the parent already has. */
+	if (src_fpu->state_ptr) {
+		int ret;
+
+		dst_fpu->state_mask = 0;
+		ret = alloc_xstate_area(dst_fpu, src_fpu->state_mask, &size);
+		if (ret)
+			return ret;
+	} else {
+		dst_fpu->state_mask = src_fpu->state_mask & ~xfeatures_mask_user_dynamic;
+		size = fpu_kernel_xstate_default_size;
+		/*
+		 * Don't let 'init optimized' areas of the XSAVE area
+		 * leak into the child task:
+		 */
+		memset(&dst_fpu->state.xsave, 0, size);
+	}
+
 	/*
 	 * If the FPU registers are not current just memcpy() the state.
 	 * Otherwise save current FPU registers directly into the child's FPU
@@ -252,7 +264,7 @@ int fpu__copy(struct task_struct *dst, struct task_struct *src)
 	 */
 	fpregs_lock();
 	if (test_thread_flag(TIF_NEED_FPU_LOAD))
-		memcpy(__xstate(dst_fpu), __xstate(src_fpu), fpu_kernel_xstate_default_size);
+		memcpy(__xstate(dst_fpu), __xstate(src_fpu), size);
 
 	else if (!copy_fpregs_to_fpstate(dst_fpu))
 		copy_kernel_to_fpregs(dst_fpu);
-- 
2.17.1