Re: [PATCH v2 00/93] TCI fixes and cleanups

[Stefan Weil]

Am 04.02.21 um 10:58 schrieb Peter Maydell:
> On Thu, 4 Feb 2021 at 01:49, Richard Henderson
> <richard.henderson@linaro.org> wrote:
> > Almost 7 years ago I detailed 5 major problems in tci[1], of
> > which three still remain:
> >
> >    * Unaligned accesses to the bytecode stream, which means
> >      that we immediately SIGBUS on any host requiring alignment.
> >    * Non-portable calls to helper functions.
> >    * Full of useless ifdefs and TODOs.
> >
> > To my mind, this means the code is unmaintained, despite what it
> > says in MAINTAINERS.  Thus tci *should* be simply removed.
> > However, every time removal is suggested, someone comes out of the
> > woodwork and says we should keep it, because it's useful for $FOO.
> Not listed, but also a problem:
>   * it's a configure-time choice, not a runtime choice


That's the feature which I also desire most. Technically it was not 
possible to have native and interpreted TCG in the same code some years 
ago when I tried to implement this, but that might have changed as TCG 
has evolved a lot. Having TCI as a runtime choice would not only require 
less CI builds but also allow new use cases for TCG testing.

I also agree with most of Richard's list of problems and appreciate the 
efforts to fix them.

I disagree on the #ifdefs which can help to understand TCG better in my 
opinion, so for me they have a useful side effect of being also 
documentation.

Most of the problems which you named above were already mentioned in the 
README for TCI from 2011.

Nevertheless I had the impression that TCI was "good enough" for those 
who used it, and it was sufficient to fix a newly used TCG opcode which 
triggered a TODO assertion from time to time. Obviously I underestimated 
Richard's desire to have a 100 % working TCI. I am sorry for that.

Technically the patch series looks reasonable. I like especially the 
disassembler. Is there a Git repository which makes pulling all changes 
easier? It would also help if the patches which were already reviewed 
were already merged in qemu master.

Regarding misaligned bytecode access, there exist two solutions. We 
could either use code which handles that correctly (I had sent a patch 
using memcpy two years ago and recently sent a V2 which uses the QEMU 
standard functions for that). Or we can align the data like it is done 
in Richard's patches. For me it is not obvious which one is better. 
While a single access is faster for aligned data, this might be 
different for sequential access on misaligned data which might profit 
from better caching of smaller bytecode.

Kind regards,

Stefan