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Date: Tue, 16 Sep 2008 21:11:12 +0200
From: "Edgar E. Iglesias" <edgar.iglesias@axis.com>
Subject: Re: [Qemu-devel] env fields as TCG variables
Message-ID: <20080916191112.GA3992@edgar.se.axis.com>
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To: Blue Swirl <blauwirbel@gmail.com>
Cc: qemu-devel <qemu-devel@nongnu.org>

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