Re: KVM: Fix the invlpg instruction emulation on AMD64

[Aurelien Jarno <aurelien-rXXEIb44qovR7s880joybQ@public.gmane.org>]

On Tue, Oct 16, 2007 at 12:12:18PM +0200, Avi Kivity wrote:
> Aurelien Jarno wrote:
> >
> >I have marked the invlpg instruction the same way as it is done in
> >kvm-37 to know what happens. I get either modrm_reg = 4 or = 6 when the
> >invlpg instruction is executed, but never = 7.
> >
> >  
> 
> Then it isn't the invlpg instruction at all.  Rather smsw (modrm_reg == 
> 4) or lmsw ( == 6).
> 
> I'm confused.
> 
> (looks)
> 
> Okay.  What we have here is total breakage when emulating an instruction 
> that uses a mod r/m encoding that actually refers to a register 
> (modrm_mod == 3).  In x86_decode_insn() we set src.type as OP_MEM, and 
> in x86_emulate_insn() we happily fetch it even though it's a register, 
> generating a fault.
> 
> It usually doesn't bite us because these instructions are directly executed.
> 
> The fix is probably to switch to OP_REG if SrcMem and ModRM and 
> modrm_mod == 3 (similarly for DstMem).
>

Ok, I have tried to implement that for SrcMem, and it works in my case.
I am able to boot FreeBSD, and I am seeing no regression for Linux or
Hurd VMs. Does the patch below looks ok to you?



KVM: Fix the invlpg instruction emulation on AMD64

The patch below correctly detects the invlpg instruction, and switch 
src.type to OP_REG. This fixes the boot of FreeBSD on an AMD64 CPU, it has
been broken since commit aa38840d3d2e0a804e628077df8d8879b496d741.

It also moves the assignation of c->src.bytes after the test as it is
not needed for the invlpg instruction.

Signed-off-by: Aurelien Jarno <aurelien-rXXEIb44qovR7s880joybQ@public.gmane.org>

diff --git a/drivers/kvm/x86_emulate.c b/drivers/kvm/x86_emulate.c
index e974ace..06e183b 100644
--- a/drivers/kvm/x86_emulate.c
+++ b/drivers/kvm/x86_emulate.c
@@ -825,12 +825,12 @@ modrm_done:
 		c->src.bytes = 4;
 		goto srcmem_common;
 	case SrcMem:
-		c->src.bytes = (c->d & ByteOp) ? 1 :
-							   c->op_bytes;
 		/* Don't fetch the address for invlpg: it could be unmapped. */
-		if (c->twobyte && c->b == 0x01
-				    && c->modrm_reg == 7)
+		if ((c->d & ModRM) && c->modrm_mod == 3) {
+			c->src.type = OP_REG;
 			break;
+		}
+		c->src.bytes = (c->d & ByteOp) ? 1 : c->op_bytes;
 srcmem_common:
 		c->src.type = OP_MEM;
 		break;


-- 
  .''`.  Aurelien Jarno	            | GPG: 1024D/F1BCDB73
 : :' :  Debian developer           | Electrical Engineer
 `. `'   aurel32-8fiUuRrzOP0dnm+yROfE0A@public.gmane.org         | aurelien-rXXEIb44qovR7s880joybQ@public.gmane.org
   `-    people.debian.org/~aurel32 | www.aurel32.net

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