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

[Aurelien Jarno <aurelien@aurel32.net>]

On Sun, Jul 25, 2010 at 01:58:51AM +0900, Jun Koi wrote:
> On Sat, Jul 24, 2010 at 11:49 AM, Aurelien Jarno <aurelien@aurel32.net> wrote:
> > 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().
> 
> This is very helpful, thanks!
> 
> Another question: as far as I can see in the code, most context is
> always synced, all the time. One exception is Eflags, which is only
> synced at the end of block.
> Is there other lazy synced context, besides EFLAGS?

It depends on the target, some of ones are also syncing the program
counter lazily.

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