Avoid save/restoring AMX registers to avoid a SPR erratum

Intel SPR erratum SPR4 says that if you trip into a vmexit while
doing FPU save/restore, your AMX register state might misbehave...
and by misbehave, I mean save all zeroes incorrectly, leading to
explosions if you restore it.

Since we're not using AMX for anything, the simple way to avoid
this is to just not save/restore those when we do anything, since
we're killing preemption of any sort across our save/restores.

If we ever decide to use AMX, it's not clear that we have any
way to mitigate this, on Linux...but I am not an expert.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Rich Ercolani <[email protected]>
Closes openzfs#14989
Closes openzfs#15168

include/os/linux/kernel/linux/simd_x86.h

@@ -147,6 +147,15 @@
 #error "Toolchain needs to support the XSAVE assembler instruction"
 #endif
 
+#ifndef XFEATURE_MASK_XTILE
+/*
+ * For kernels where this doesn't exist yet, we still don't want to break
+ * by save/restoring this broken nonsense.
+ * See issue #14989 or Intel errata SPR4 for why
+ */
+#define	XFEATURE_MASK_XTILE	0x60000
+#endif
+
 #include <linux/mm.h>
 #include <linux/slab.h>
 
@@ -315,18 +324,18 @@ kfpu_begin(void)
 	uint8_t *state = zfs_kfpu_fpregs[smp_processor_id()];
 #if defined(HAVE_XSAVES)
 	if (static_cpu_has(X86_FEATURE_XSAVES)) {
-		kfpu_do_xsave("xsaves", state, ~0);
+		kfpu_do_xsave("xsaves", state, ~XFEATURE_MASK_XTILE);
 		return;
 	}
 #endif
 #if defined(HAVE_XSAVEOPT)
 	if (static_cpu_has(X86_FEATURE_XSAVEOPT)) {
-		kfpu_do_xsave("xsaveopt", state, ~0);
+		kfpu_do_xsave("xsaveopt", state, ~XFEATURE_MASK_XTILE);
 		return;
 	}
 #endif
 	if (static_cpu_has(X86_FEATURE_XSAVE)) {
-		kfpu_do_xsave("xsave", state, ~0);
+		kfpu_do_xsave("xsave", state, ~XFEATURE_MASK_XTILE);
 	} else if (static_cpu_has(X86_FEATURE_FXSR)) {
 		kfpu_save_fxsr(state);
 	} else {
@@ -376,12 +385,12 @@ kfpu_end(void)
 	uint8_t  *state = zfs_kfpu_fpregs[smp_processor_id()];
 #if defined(HAVE_XSAVES)
 	if (static_cpu_has(X86_FEATURE_XSAVES)) {
-		kfpu_do_xrstor("xrstors", state, ~0);
+		kfpu_do_xrstor("xrstors", state, ~XFEATURE_MASK_XTILE);
 		goto out;
 	}
 #endif
 	if (static_cpu_has(X86_FEATURE_XSAVE)) {
-		kfpu_do_xrstor("xrstor", state, ~0);
+		kfpu_do_xrstor("xrstor", state, ~XFEATURE_MASK_XTILE);
 	} else if (static_cpu_has(X86_FEATURE_FXSR)) {
 		kfpu_restore_fxsr(state);
 	} else {