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

[Jun Koi]

On Sun, Jul 25, 2010 at 7:57 AM, Aurelien Jarno <address@hidden> wrote:
> On Sun, Jul 25, 2010 at 01:58:51AM +0900, Jun Koi wrote:
>> On Sat, Jul 24, 2010 at 11:49 AM, Aurelien Jarno <address@hidden> 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 <address@hidden> 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.
>

ok, now i see that on x86, only EFLAGS is lazy sync.

many thanks,
J