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

[Palmer Dabbelt]

On Wed, 30 Mar 2022 22:13:39 PDT (-0700), alistair23 at gmail.com wrote:
> On Thu, Mar 31, 2022 at 2:36 PM Palmer Dabbelt <palmer at dabbelt.com> wrote:
>>
>> On Wed, 30 Mar 2022 20:23:21 PDT (-0700), alistair23 at gmail.com wrote:
>> > On Thu, Mar 31, 2022 at 3:11 AM Idan Horowitz <idan.horowitz at gmail.com> wrote:
>> >>
>> >> On Wed, 30 Mar 2022 at 19:11, Palmer Dabbelt <palmer at 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!