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

[Nadav Amit]

Gleb,

On 4/20/14, 12:26 PM, Gleb Natapov wrote:
> 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@xxxxxxxxxxxxxxxxx>
> > ---
> > :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.
It appears to be so.
Results of 32-bit operations are implicitly zero extended to 64-bit 
values, and this appears to apply to all 32 bit operations, including 
implicit ones. Therefore it seems to apply to all these operations.

> >   }
> >
> >   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?
The observed behaviour of the Sandy-Bridge I use, is that even if ECX is 
zero on the first iteration, the high half of RCX is zeroed. Therefore, 
this is a different case, which was not covered in 
register_address_increment. I agree it is totally undocumented.
Following your previous comment - I may have missed the case in which 
loop instruction is executed with ECX = 0 while RCX != 0 and the address 
size is 32 bit. I will test this case soon (yet, it is lower on my 
priority list).

Nadav

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