Re: [Qemu-devel] Question on save_globals() in TCG

[Aurelien Jarno <aurelien@aurel32.net>]

On Sat, Jul 24, 2010 at 09:35:44AM +0900, Jun Koi wrote:
> On Sat, Jul 24, 2010 at 2:53 AM, Aurelien Jarno <aurelien@aurel32.net> wrote:
> > On Fri, Jul 23, 2010 at 08:59:54PM +0900, Jun Koi wrote:
> >> Hi,
> >>
> >> I am looking at the save_globals() of TCG code, and it seems this
> >> function saves regular registers like EAX, ..., EDI back to CPU state.
> >>
> >> But I am not sure if it also saves value of other registers, like
> >> EFlags, Segments, CR*, DR*, ... (?)
> >> From what I saw, it doesnt seem to do so. Is it correct?
> >>
> >
> > save_globals() is run before calling a function that can trigger a CPU
> > exception, to make sure that in that case all TCG variables are synced
> > with the CPU state.
> 
> Is it correct? I always assume that save_globals() is also called at
> end of each block.

Correct also here.

> >
> > Given the CPU state only uses "normal" registers, there is no need to
> > save the other registers.
> 
> Why do you say that CPU state includes only normal registers, given
> that, like on x86, CPUState has also segs[], cr[] and dr[]?
>
>
>
> Another question: if save_globals() only saves regular registers,
> where Qemu saves other registers  like segs[], cr[]? Or do they always
> sync, all the time?

It's actually the same question. You are mixing host and target
registers. save_globals() only saves "normal" host registers. Host 
registers can then contain whatever registers from the target, and
more precisely the one declared as globals.


> Finally, how about Eflags? I remember that Eflags is "lazy sync", but
> does it sync at end of each block?
> 

It's the same. Eflags are actually stored in a lazy way in cc_*
globals. They are therefore synced with save_globals().


-- 
Aurelien Jarno	                        GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net