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

[Jun Koi]

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?

Thanks,
Jun