Re: [PATCH -v2] kvm: Emulate MOVBE

[Borislav Petkov <bp@alien8.de>]

On Tue, Apr 16, 2013 at 01:36:00PM +0200, Paolo Bonzini wrote:
> Il 14/04/2013 23:02, Borislav Petkov ha scritto:
> > 	*(u16 *)&ctxt->dst.val = swab16((u16)ctxt->src.val);
> > 
> > 	movzwl  112(%rdi), %eax # ctxt_5(D)->src.D.27823.val, tmp82
> > 	rolw    $8, %ax #, tmp82
> > 	movw    %ax, 240(%rdi)  # tmp82, MEM[(u16 *)ctxt_5(D) + 240B]
> 
> I think this breaks the C aliasing rules.
> 
> Using valptr is okay because it is a char.

Yep, good catch.

We normally build with -fno-strict-aliasing but when I change that, gcc
catches it:

arch/x86/kvm/emulate.c: In function ‘em_movbe’:
arch/x86/kvm/emulate.c:3121:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]

However, it screams even louder about the valptr variant

	*(u16 *)ctxt->dst.valptr = swab16(*(u16 *)ctxt->src.valptr);

too:

arch/x86/kvm/emulate.c: In function ‘em_movbe’:
arch/x86/kvm/emulate.c:3117:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
arch/x86/kvm/emulate.c:3117:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
arch/x86/kvm/emulate.c:3117:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
arch/x86/kvm/emulate.c:3117:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
arch/x86/kvm/emulate.c:3117:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]

We probably need something with copying values to a temp variable or so.

Thanks.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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