Re: [PATCH 2/2] accel/tcg: add assert() check in tb_invalidate_phys_page_range__locked()

[BALATON Zoltan]

On Fri, 30 Jun 2023, Michael Tokarev wrote:
> 29.06.2023 11:25, Mark Cave-Ayland wrote:
> > Add an assert() check in tb_invalidate_phys_page_range__locked() to ensure 
> > that
> > both the start and last addresses are within the same target page. Note 
> > that
> > due to performance concerns the check is only enabled when QEMU is 
> > configured
> > with --enable-debug-tcg.
>
> Performance concerns? That's two ANDs and on compare, - is it really that 
> performance
> critical?
>
> I'm just asking, I dunno.

If something is called frequently enough any small computaion can add up. 
In this case invalidating pages is probably a performance hit already and 
hopefully does not happen too often but then it's a good idea not to make 
it worse. As this is not something that should or could normally happen 
and only checks for programming errors I think it's good idea to only do 
it when debugging.

Regards,
BALATON Zoltan

> Thanks,
>
> /mjt
>
> > Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
> > ---
> >   accel/tcg/tb-maint.c | 4 ++++
> >   1 file changed, 4 insertions(+)
> >
> > diff --git a/accel/tcg/tb-maint.c b/accel/tcg/tb-maint.c
> > index 33ea1aadd1..8cd730dcb0 100644
> > --- a/accel/tcg/tb-maint.c
> > +++ b/accel/tcg/tb-maint.c
> > @@ -1092,6 +1092,10 @@ tb_invalidate_phys_page_range__locked(struct 
> > page_collection *pages,
> >       TranslationBlock *current_tb = retaddr ? tcg_tb_lookup(retaddr) : 
> > NULL;
> >   #endif /* TARGET_HAS_PRECISE_SMC */
> >   +#ifdef CONFIG_DEBUG_TCG
> > +    assert((last & TARGET_PAGE_MASK) == (start & TARGET_PAGE_MASK));
> > +#endif
> > +
> >       /*
> >        * We remove all the TBs in the range [start, last].
> >        * XXX: see if in some cases it could be faster to invalidate all the 
> > code