Re: [PATCH 4/5] KVM: x86: RSI/RDI/RCX are zero-extended when affected by string ops

[Gleb Natapov <gleb@kernel.org>]

On Fri, Apr 18, 2014 at 07:11:33AM +0300, Nadav Amit wrote:
> When using address-size override prefix with string instructions in long-mode,
> ESI/EDI/ECX are zero extended if they are affected by the instruction
> (incremented/decremented).  Currently, the KVM emulator does not do so.
> 
> In addition, although it is not well-documented, when address override prefix
> is used with REP-string instruction, RCX high half is zeroed even if ECX was
> zero on the first iteration. Therefore, the emulator should clear the upper
> part of RCX in this case, as x86 CPUs do.
> 
> Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
> ---
> :100644 100644 69e2636... a69ed67... M	arch/x86/kvm/emulate.c
>  arch/x86/kvm/emulate.c |    4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 69e2636..a69ed67 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -491,6 +491,8 @@ register_address_increment(struct x86_emulate_ctxt *ctxt, unsigned long *reg, in
>  	else
>  		mask = ad_mask(ctxt);
>  	masked_increment(reg, mask, inc);
> +	if (ctxt->ad_bytes == 4)
> +		*reg &= 0xffffffff;
*reg=(u32)*reg; and you can do it inside else part.

register_address_increment() is used also by jmp_rel and loop instructions,
is this correct for both of those too? Probably yes.

>  }
>  
>  static void rsp_increment(struct x86_emulate_ctxt *ctxt, int inc)
> @@ -4567,6 +4569,8 @@ int x86_emulate_insn(struct x86_emulate_ctxt *ctxt)
>  	if (ctxt->rep_prefix && (ctxt->d & String)) {
>  		/* All REP prefixes have the same first termination condition */
>  		if (address_mask(ctxt, reg_read(ctxt, VCPU_REGS_RCX)) == 0) {
> +			if (ctxt->ad_bytes == 4)
> +				*reg_write(ctxt, VCPU_REGS_RCX) = 0;
Does zero extension happens even if ECX was zero at the beginning on an instruction or only during
ECX modification. If later it is already covered in register_address_increment, no?

>  			ctxt->eip = ctxt->_eip;
>  			goto done;
>  		}
> -- 
> 1.7.10.4
> 

--
			Gleb.