Re: [Qemu-devel] [PATCH v3 13/34] tcg: Add atomic helpers

[Nikunj A Dadhania]

Richard Henderson <address@hidden> writes:

> On 09/12/2016 06:47 AM, Alex Bennée wrote:
>>> > +    /* Notice an IO access, or a notdirty page.  */
>>> > +    if (unlikely(tlb_addr & ~TARGET_PAGE_MASK)) {
>>> > +        /* There's really nothing that can be done to
>>> > +           support this apart from stop-the-world.  */
>>> > +        goto stop_the_world;
>> We are also triggering on TLB_NOTDIRTY here in the case where a
>> conditional write is the first write to a page. I don't know if a
>> stop_the_world is required at this point but we will need to ensure we
>> clear bits as notdirty_mem_write() does.
>> 
>
> You're quite right that we could probably special-case TLB_NOTDIRTY here such
> that (1) we needn't leave the cpu loop, and (2) needn't utilize the actual
> "write" part of notdirty_mem_write; just set the bits then fall through to the
> actual atomic instruction below.

I do hit this case with ppc64, where I see that its the first write to
the page and it exits from this every time, causing the kernel to print
soft-lockups.

Can we add the special case here for NOTDIRTY and set the page as dirty
and return successfully?

Regards
Nikunj