Re: [Qemu-devel] env fields as TCG variables

[Edgar E. Iglesias]

On Tue, Sep 16, 2008 at 09:37:41PM +0300, Blue Swirl wrote:
> I wonder if the following bug scenario is possible.
> 
> Some of the env fields are accessible as TCG variables, like cpu_fsr
> for Sparc floating point status register (%fsr), stored in the env
> structure field env->fsr. When cpu_fsr is used in the generated code,
> a host CPU register is allocated for this task. But env->fsr can be
> modified directly in op_helper.c code. Isn't it then possible that the
> register (which stores cpu_fsr) and the structure field in memory
> (env->fsr) get out of sync?
> 
> If this is true, no code outside the generated code should ever modify
> or even read env fields that are also available as TCG variables. Or
> do I miss something?

Hello,

IIRC:
TCG writebacks before calling helpers. There used to be a bug with
qemu_ld/qemu_st like the one you described but it was fixed a long
time ago. I ran into that one with CRIS and avoided it for a while
by calling a dummy helper before load/stores.

Best regards