[PATCH] APIC: Read Error Status Register correctly

[Richard Weinberger]

Currently we do a read, a dummy write and a final read to fetch the error code.
The value from the final read is taken.
This is not the recommended way and leads to corrupted/lost ESR values.

Intel(c) 64 and IA-32 Architectures Software Developer's Manual,
Combined Volumes 1, 2ABC, 3ABC, Section 10.5.3 states:
>>Before attempt to read from the ESR, software should first write to it.
(The value written does not affect the values read subsequently;
only zero may be written in x2APIC mode.) This write clears
any previously logged errors and updates the ESR with any errors
detected since the last write to the ESR.
This write also rearms the APIC error interrupt triggering mechanism.<<

This patch removes the first read such that we are conform with the manual.

On my (very old) Pentium MMX SMP system this patch fixes the issue that
APIC errors a) are not always reported and b) are reported with false
error numbers.

Signed-off-by: Richard Weinberger <richard@xxxxxx>
---
 arch/x86/kernel/apic/apic.c | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index d278736..4ec1dd6 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -1968,7 +1968,7 @@ __visible void smp_trace_spurious_interrupt(struct pt_regs *regs)
  */
 static inline void __smp_error_interrupt(struct pt_regs *regs)
 {
-	u32 v0, v1;
+	u32 v;
 	u32 i = 0;
 	static const char * const error_interrupt_reason[] = {
 		"Send CS error",		/* APIC Error Bit 0 */
@@ -1982,21 +1982,20 @@ static inline void __smp_error_interrupt(struct pt_regs *regs)
 	};
 
 	/* First tickle the hardware, only then report what went on. -- REW */
-	v0 = apic_read(APIC_ESR);
 	apic_write(APIC_ESR, 0);
-	v1 = apic_read(APIC_ESR);
+	v = apic_read(APIC_ESR);
 	ack_APIC_irq();
 	atomic_inc(&irq_err_count);
 
-	apic_printk(APIC_DEBUG, KERN_DEBUG "APIC error on CPU%d: %02x(%02x)",
-		    smp_processor_id(), v0 , v1);
+	apic_printk(APIC_DEBUG, KERN_DEBUG "APIC error on CPU%d: %02x",
+		    smp_processor_id(), v);
 
-	v1 = v1 & 0xff;
-	while (v1) {
-		if (v1 & 0x1)
+	v &= 0xff;
+	while (v) {
+		if (v & 0x1)
 			apic_printk(APIC_DEBUG, KERN_CONT " : %s", error_interrupt_reason[i]);
 		i++;
-		v1 >>= 1;
+		v >>= 1;
 	}
 
 	apic_printk(APIC_DEBUG, KERN_CONT "\n");
-- 
1.8.4.2

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