x86/fpu/xstate: Don't assume the first zero xfeatures zero bit means the end

The current xstate code in setup_xstate_features() assumes that
the first zero bit means the end of xfeatures - but that is not
so, the SDM clearly states that an arbitrary set of xfeatures
might be enabled - and it is also clear from the description
of the compaction feature that holes are possible:

  "13-6 Vol. 1MANAGING STATE USING THE XSAVE FEATURE SET
  [...]

  Compacted format. Each state component i (i ≥ 2) is located at a byte
  offset from the base address of the XSAVE area based on the XCOMP_BV
  field in the XSAVE header:

  — If XCOMP_BV[i] = 0, state component i is not in the XSAVE area.

  — If XCOMP_BV[i] = 1, the following items apply:

  • If XCOMP_BV[j] = 0 for every j, 2 ≤ j < i, state component i is
    located at a byte offset 576 from the base address of the XSAVE
    area. (This item applies if i is the first bit set in bits 62:2 of
    the XCOMP_BV; it implies that state component i is located at the
    beginning of the extended region.)

  • Otherwise, let j, 2 ≤ j < i, be the greatest value such that
    XCOMP_BV[j] = 1. Then state component i is located at a byte offset
    X from the location of state component j, where X is the number of
    bytes required for state component j as enumerated in
    CPUID.(EAX=0DH,ECX=j):EAX. (This item implies that state component i
    immediately follows the preceding state component whose bit is set
    in XCOMP_BV.)"

So don't assume that the first zero xfeatures bit means the end of
all xfeatures - iterate through all of them.

I'm not aware of hardware that triggers this currently.

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/fpu/xstate.c

@@ -168,26 +168,27 @@ void fpu__init_cpu_xstate(void)
 }
 
 /*
- * Record the offsets and sizes of different state managed by the xsave
- * memory layout.
+ * Record the offsets and sizes of various xstates contained
+ * in the XSAVE state memory layout.
+ *
+ * ( Note that certain features might be non-present, for them
+ *   we'll have 0 offset and 0 size. )
  */
 static void __init setup_xstate_features(void)
 {
-	int eax, ebx, ecx, edx, leaf = 0x2;
+	u32 eax, ebx, ecx, edx, leaf;
 
 	xfeatures_nr = fls64(xfeatures_mask);
 
-	do {
+	for (leaf = 2; leaf < xfeatures_nr; leaf++) {
 		cpuid_count(XSTATE_CPUID, leaf, &eax, &ebx, &ecx, &edx);
 
-		if (eax == 0)
-			break;
-
 		xstate_offsets[leaf] = ebx;
 		xstate_sizes[leaf] = eax;
 
+		printk(KERN_INFO "x86/fpu: xstate_offset[%d]: %04x, xstate_sizes[%d]: %04x\n", leaf, ebx, leaf, eax);
 		leaf++;
-	} while (1);
+	}
 }
 
 static void print_xstate_feature(u64 xstate_mask)