Re: [PATCH 1/2] target/s390x: Define TARGET_HAS_PRECISE_SMC

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Mon, 2023-08-07 at 17:31 +0200, David Hildenbrand wrote:
> On 07.08.23 13:48, Ilya Leoshkevich wrote:
> > PoP (Sequence of Storage References -> Instruction Fetching) says:
> > 
> >      ... if a store that is conceptually earlier is
> >      made by the same CPU using the same effective
> >      address as that by which the instruction is subse-
> >      quently fetched, the updated information is obtained ...
> > 
> > QEMU already has support for this in the common code; enable it for
> > s390x.
> 
> 
> Figuring out what TARGET_HAS_PRECISE_SMC is all about, I only learned
> from git history
> 
> commit d720b93d0bcfe1beb729245b9ed1e5f071a24bd5
> Author: Fabrice Bellard <fabrice@bellard.org>
> Date:   Sun Apr 25 17:57:43 2004 +0000
> 
>      precise self modifying code support
> 
> 
>      git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@745 
> c046a42c-6fe2-441c-8c8c-71466251a162
> 
> 
> 
> AFAIU, precise SMC is stricter compared to what we have right now. So
> i 
> suspect that this patch is actually fixing SMC behavior: for example,
> when a basic block ends up modifying itself.
> 
> Were there any BUG reports? (does patch #2 test for that and can 
> reproduce the original issue?)

There were no bug reports, I found this issue with fuzzing.

Patch #2 tests a TB modifying itself.
Reverting this commit makes the test fail.

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