Re: Re: [PATCH] target/riscv: Exit current TB after an sfence.vma

[Palmer Dabbelt]

On Wed, 30 Mar 2022 22:13:39 PDT (-0700), alistair23@gmail.com wrote:
> On Thu, Mar 31, 2022 at 2:36 PM Palmer Dabbelt <palmer@dabbelt.com> wrote:
> > On Wed, 30 Mar 2022 20:23:21 PDT (-0700), alistair23@gmail.com wrote:
> > > On Thu, Mar 31, 2022 at 3:11 AM Idan Horowitz <idan.horowitz@gmail.com> wrote:
> > >>
> > >> On Wed, 30 Mar 2022 at 19:11, Palmer Dabbelt <palmer@dabbelt.com> wrote:
> > >> >
> > >> >
> > >> > Presumably you mean "revert" here?  That might be the right way to go,
> > >> > just to avoid breaking users (even if we fix the kernel bug, it'll take
> > >> > a while to get everyone to update).  That said, this smells like the
> > >> > sort of thing that's going to crop up at arbitrary times in dynamic
> > >> > systems so while a revert looks like it'd work around the boot issue we
> > >> > might be making more headaches for folks down the road.
> > >> >
> > >>
> > >> The opposite in fact, I did not suggest to revert it, but rather undo
> > >> the revert (as Alistair already removed it from the apply-next tree),
> > >> since my original patch fixes buggy behaviour that is blocking the
> > >> testing of some embedded software on QEMU.
> >
> > Ah, sorry -- the QEMU tree I was looking at still had the patch in
> > there, must have just been an old one.
> >
> > > So, this is a little tricky.
> > >
> > > We want to apply the fix, but that will break current users.
> > >
> > > Once the fix is merged into Linux we can apply it here. That should
> > > hopefully be right at the start of the 7.1 QEMU development window,
> > > which should give time for the fix to propagate into stable kernels
> > > and not break too many people by the time QEMU is released.
> >
> > If you think this is a Linux bug then that makes sense, but I think this
> > is a QEMU bug -- I sent a patch, not sure if it went through as it didn't
> > make it to lore.
>
> Ah whoops. I saw the patch but didn't read it, then I assumed it was a
> Linux bug from your diff earlier.

No problem, that was the first thing I sent in the morning so I doubt it 
made any sense.

> > I also think the bug will manifest without the TB exit patch, maybe in
> > single-step mode and definately if we happen to exit the TB at that
> > point for other reasons.  Assuming my reasoning is correct in that
> > patch, we may also be hitting this as arbitrary corruption anywhere.
> > I'd started to write up a "QEMU errata" Linux patch for this, but then
> > convinced myself that just adding the sfence.vma was insufficient.
>
> Yeah, looking at it now I agree, I'll send a PR for 7.0.

Thanks!