[RFC PATCH] x86/fpu/xstate: Add more diagnostic information on inconsistent xstate sizes

[Fenghua Yu]

A warning is emitted when xstate sizes are found inconsistent.
But the warning message doesn't show enough information to diagnose
the issue.

Provide more detailed xstate size information to help debug the issue.
As a hypothetical example, on a platform that may report incorrect
xstate size in CPUID.0xd.1:EBX, the diagnostic information after
the warning will show:
[    0.000000] x86/fpu: max_features=0x407
[    0.000000] x86/fpu: xstate_offset[2]:  576, xstate_sizes[2]:  256
[    0.000000] x86/fpu: xstate_offset[10]: 832, xstate_sizes[10]:    8
[    0.000000] x86/fpu: total size: 840 bytes
[    0.000000] x86/fpu: XCR0=0x7, IA32_XSS=0x400
[    0.000000] x86/fpu: kernel_size from CPUID.0xd.0x1:EBX: 576 bytes

XCR0 | IA32_XSS is 0x407 which is consistent with max_features.
CPUID.0xd.0x1:EBX should report the size of the xsave area
containing xstate components corresponding to bits set in
XCR0 | IA32_XSS. But it only reports 576 bytes of xsave area which
doesn't count xstate sizes for AVX (offset 2 and 256 bytes) and
PASID (offset 10 and 8 bytes). This confirms that the platform
reports xstate size incorrectly through the CPUID bits.

Suggest-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Tested-by: Chintan M Patel <chintan.m.patel@xxxxxxxxx>
Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
---
 arch/x86/kernel/fpu/xstate.c | 33 +++++++++++++++++++++++++++++++--
 1 file changed, 31 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 0bab497c9436..5f27fcdc6c90 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -602,8 +602,37 @@ static bool __init paranoid_xstate_size_valid(unsigned int kernel_size)
 		}
 	}
 	size = xstate_calculate_size(fpu_kernel_cfg.max_features, compacted);
-	XSTATE_WARN_ON(size != kernel_size,
-		       "size %u != kernel_size %u\n", size, kernel_size);
+	if (size != kernel_size) {
+		u64 xcr0, ia32_xss;
+
+		XSTATE_WARN_ON(1, "size %u != kernel_size %u\n",
+			       size, kernel_size);
+
+		/* Show more information to help diagnose the size issue. */
+		pr_info("x86/fpu: max_features=0x%llx\n",
+			fpu_kernel_cfg.max_features);
+		print_xstate_offset_size();
+		pr_info("x86/fpu: total size: %u bytes\n", size);
+		xcr0 = xgetbv(XCR_XFEATURE_ENABLED_MASK);
+		if (compacted) {
+			rdmsrl(MSR_IA32_XSS, ia32_xss);
+			pr_info("x86/fpu: XCR0=0x%llx, IA32_XSS=0x%llx\n",
+				xcr0, ia32_xss);
+		} else {
+			pr_info("x86/fpu: XCR0=0x%llx\n", xcr0);
+		}
+		/*
+		 * In compact case, CPUID.0xd.0x1:EBX reports the size of
+		 * the XSAVE size containing all the state components
+		 * corresponding to bits set in XCR0 | IA32_XSS.
+		 *
+		 * Otherwise, CPUID.0xd.0x0:EBX reports the size of an XSAVE
+		 * area containing all the *user* state components
+		 * corresponding to bits set in XCR0.
+		 */
+		pr_info("x86/fpu: kernel_size from CPUID.0xd.0x%x:EBX: %u bytes\n",
+			compacted ? 1 : 0, kernel_size);
+	}
 	return size == kernel_size;
 }
 
-- 
2.37.1