Re: [PATCH 2/2] KVM: x86 emulator: add decoding of CMPXCHG8B dst operand.

[Avi Kivity <avi@redhat.com>]

On 03/21/2010 01:08 PM, Gleb Natapov wrote:
> Decode CMPXCHG8B destination operand in decoding stage. Fixes regression
> introduced by "If LOCK prefix is used dest arg should be memory" commit.
> This commit relies on dst operand be decoded at the beginning of an
> instruction emulation.
>    

> @@ -1719,15 +1719,12 @@ static inline int emulate_grp9(struct x86_emulate_ctxt *ctxt,
>   		c->regs[VCPU_REGS_RAX] = (u32) (old>>  0);
>   		c->regs[VCPU_REGS_RDX] = (u32) (old>>  32);
>   		ctxt->eflags&= ~EFLG_ZF;
> -
>   	} else {
> -		new = ((u64)c->regs[VCPU_REGS_RCX]<<  32) |
> +		c->dst.val = ((u64)c->regs[VCPU_REGS_RCX]<<  32) |
>   		       (u32) c->regs[VCPU_REGS_RBX];
>
> -		rc = ops->cmpxchg_emulated(c->modrm_ea,&old,&new, 8, ctxt->vcpu);
> -		if (rc != X86EMUL_CONTINUE)
> -			return rc;
>   		ctxt->eflags |= EFLG_ZF;
> +		c->lock_prefix = 1;
>    

Why is this bit needed?  cmpxchg64b without lock is valid and racy, but 
the guest may know it is safe.

-- 
error compiling committee.c: too many arguments to function