Re: [kvm-devel] [BUG] Oops with KVM-27

[Luca Tettamanti <kronos.it@gmail.com>]

Il Fri, Jun 15, 2007 at 12:53:24AM +0200, Luca Tettamanti ha scritto: 
> Il Thu, Jun 14, 2007 at 11:26:29AM +0300, Avi Kivity ha scritto: 
> > Luca Tettamanti wrote:
> > >With GOOD_APIC apic_read_around is a nop, while apic_write_around is a
> > >normal write. With !GOOD_APIC apic_write_around writes to the APIC reg
> > >using xchg. With !GOOD_APIC and this patch:
> > >
> > >--- include/asm-i386/apic.h~	2007-04-26 05:08:32.000000000 +0200
> > >+++ include/asm-i386/apic.h	2007-06-13 22:35:00.000000000 +0200
> > >@@ -56,7 +56,8 @@
> > > static __inline fastcall void native_apic_write_atomic(unsigned long reg,
> > > 						       unsigned long v)
> > > {
> > >-	xchg((volatile unsigned long *)(APIC_BASE+reg), v);
> > >+//	xchg((volatile unsigned long *)(APIC_BASE+reg), v);
> > >+	*((volatile unsigned long *)(APIC_BASE+reg)) = v;
> > > }
> > > 
> > > static __inline fastcall unsigned long native_apic_read(unsigned long reg)
> > >
> > >The kernel boots fine. 
> > >  
> > 
> > Looking at the xchg emulation code, it seems fine, but clearly it 
> > isn't.
> 
> Btw, I've put a printk in x86_emulate.c, where it prepares the operands
> for the xchg operations: all the write_atomic are hitting this point,
> so the write is lost somewhere in cmpxchg_emulated->write_emulated.

Got it!
The emulator skips the writeback if the old value is unchanged, so the
apic doesn't see the write.

Forcing the writeback:

-      if ((d & Mov) || (dst.orig_val != dst.val)) {
-      if ((d & Mov) || (dst.orig_val != dst.val) || isxchg) {

seems to fix the issue :D I'm not sure that fix is correct though. This
is the patch I'm using:


--- a/kernel/x86_emulate.c	2007-06-03 10:31:15.000000000 +0200
+++ b/kernel/x86_emulate.c	2007-06-15 01:12:12.000000000 +0200
@@ -486,6 +486,7 @@
 	unsigned long _regs[NR_VCPU_REGS];
 	unsigned long _eip = ctxt->vcpu->rip, _eflags = ctxt->eflags;
 	unsigned long modrm_val = 0;
+	int isxchg = 0;
 
 	memcpy(_regs, ctxt->vcpu->regs, sizeof _regs);
 
@@ -912,6 +913,7 @@
 		break;
 	case 0x86 ... 0x87:	/* xchg */
 		/* Write back the register source. */
+		isxchg = 1;
 		switch (dst.bytes) {
 		case 1:
 			*(u8 *) src.ptr = (u8) dst.val;
@@ -1056,7 +1058,7 @@
 	}
 
 writeback:
-	if ((d & Mov) || (dst.orig_val != dst.val)) {
+	if ((d & Mov) || (dst.orig_val != dst.val) || isxchg) {
 		switch (dst.type) {
 		case OP_REG:
 			/* The 4-byte case *is* correct: in 64-bit mode we zero-extend. */

Luca
-- 
"Su cio` di cui non si puo` parlare e` bene tacere".
 Ludwig Wittgenstein