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From: Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH -v2] kvm: Emulate MOVBE
Date: Mon, 22 Apr 2013 11:52:03 +0200
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Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Andre Przywara <andre@andrep.de>, kvm@vger.kernel.org,
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To: Gleb Natapov <gleb@redhat.com>
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On Mon, Apr 22, 2013 at 12:42:46PM +0300, Gleb Natapov wrote:
> > Btw, I wanted to ask: when kvm commits the results, does it look at
> > ctxt->op_bytes to know exactly how many bytes to write to the guest?
> > Because if it does, we can save ourselves the trouble here.
> > 
> > Or does it simply write both the full sizeof(unsigned long) bytes of
> > ->src.val and ->dst.val to the guest?
> > 
> No, it does this in case of register operand:
> 
> static void write_register_operand(struct operand *op)
> {
>         /* The 4-byte case *is* correct: in 64-bit mode we zero-extend.  */
>         switch (op->bytes) {
>         case 1:
>                 *(u8 *)op->addr.reg = (u8)op->val;
>                 break;
>         case 2:
>                 *(u16 *)op->addr.reg = (u16)op->val;
>                 break;
>         case 4:
>                 *op->addr.reg = (u32)op->val;
>                 break;  /* 64b: zero-extend */
>         case 8:
>                 *op->addr.reg = op->val;
>                 break;
>         }
> }

Ok, and for OP_MEM it does look at ctxt->dst.bytes in writeback(),
AFAICT. And I see other emulated instructions like POPF, for example, do
this:

	ctxt->dst.bytes = ctxt->op_bytes;

Which means, we can drop all the bullshit in em_movbe and even destroy
some of the bytes in dst.val but only write out the correct ones. Which
means, a simpler code and a lot less jumping through hoops.

Would that be the more accepted practice?

Thanks.

-- 
Regards/Gruss,
    Boris.

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