Re: [patch] fix zero extending for mmio ld1/2/4 emulation in KVM

[Jes Sorensen <jes@sgi.com>]

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Matthew Chapman wrote:
> Jes,
> 
> Glad you tracked it down.  Can I suggest rather than using memcpy, a
> more efficient way might be something like...
> 
> #define ZERO_EXTEND(x,bits) ((x) & (~0UL >> (64-(bits))))
> 
> *dest = ZERO_EXTEND(p->u.ioreq.data, 8*s);

Much nicer indeed!

Here's a pretty version - Tony will you apply this one instead.

Cheers,
Jes



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Only copy in the data actually requested by the instruction emulation
and zero pad the destination register first. This avoids the problem
where emulated mmio access got garbled data from ld2.acq instructions
in the vga console driver.

Signed-off-by: Jes Sorensen <jes@sgi.com>

---
 arch/ia64/kvm/mmio.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

Index: linux-2.6.git/arch/ia64/kvm/mmio.c
===================================================================
--- linux-2.6.git.orig/arch/ia64/kvm/mmio.c
+++ linux-2.6.git/arch/ia64/kvm/mmio.c
@@ -159,7 +159,8 @@
 
 	if (p->u.ioreq.state == STATE_IORESP_READY) {
 		if (dir == IOREQ_READ)
-			*dest = p->u.ioreq.data;
+			/* it's necessary to ensure zero extending */
+			*dest = p->u.ioreq.data & (~0UL >> (64-(s*8)));
 	} else
 		panic_vm(vcpu);
 out: