Re: [Qemu-devel] Call Trace for QEMU functions

[Naman patel <naman321@gmail.com>]

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On Fri, Jul 31, 2015 at 3:32 PM, Peter Maydell <peter.maydell@linaro.org>
wrote:

> On 31 July 2015 at 10:29, Naman patel <naman321@gmail.com> wrote:
> > Can someone explain me what happens when a guest OS calls "invlpg" on say
> > page swap out or a context switch? What exactly is the call flow and how
> > QEMU handles this instruction?
>
> When we see the instruction during translation, we emit code
> which will
>
​In which function does the translation takes place?​


>  * get the argument to invlpg (extracting it from the
> relevant register, adding any constant offset required for
> the addressing mode, etc)
>
​Where exactly is it done?​


>  * make a call to helper_inlvpg().
> (target-i386/translate.c has the C code that does this codegen --
> search for invlpg)
>
​Thanks I got this function.​


>
> Later when that generated code is run, we call the helper, which
> does what it needs to do (in this case flush a page from QEMU's
> TLB cache). When the call returns we'll carry on executing whatever
> guest instruction comes next.
>
> > Also is there anyway QEMU can send some data
> > back to the guest OS?
>
> ​​
> INVLPG is just an x86 instruction that performs an operation;
> it doesn't modify any registers or flags. So there's no way
> for it to tell the guest OS anything.
>
​True that ​
​
INVLPG only invalidates a TLB entry but in general how can one send a data
structure(DS) back to guest OS from QEMU? I want to write my own function
that can send a DS back to the guest OS. Are there any similar functions
already present that sends back DS to guest? Or is it feasible?

>
> -- PMM
>


​Thanks a lot for this information. It helped a lot.

Regards,
Naman​

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