Re: [PATCH v4 2/5] docs/about/deprecated: Deprecate the qemu-system-i386 binary

[Daniel P. Berrangé <berrange@xxxxxxxxxx>]

On Mon, Mar 06, 2023 at 02:48:16PM +0100, Thomas Huth wrote:
> On 06/03/2023 10.27, Daniel P. Berrangé wrote:
> > On Mon, Mar 06, 2023 at 09:46:55AM +0100, Thomas Huth wrote:
> > > [...] If a 32-bit CPU guest
> > > +environment should be enforced, you can switch off the "long mode" CPU
> > > +flag, e.g. with ``-cpu max,lm=off``.
> > 
> > I had the idea to check this today and this is not quite sufficient,
> [...]
> > A further difference is that qemy-system-i686 does not appear to enable
> > the 'syscall' flag, but I've not figured out where that difference is
> > coming from in the code.
> 
> I think I just spotted this by accident in target/i386/cpu.c
> around line 637:
> 
> #ifdef TARGET_X86_64
> #define TCG_EXT2_X86_64_FEATURES (CPUID_EXT2_SYSCALL | CPUID_EXT2_LM)
> #else
> #define TCG_EXT2_X86_64_FEATURES 0
> #endif

Hmm, so right now the difference between qemu-system-i386 and
qemu-system-x86_64 is based on compile time conditionals. So we
have the burden of building everything twice and also a burden
of testing everything twice.

If we eliminate qemu-system-i386 we get rid of our own burden,
but users/mgmt apps need to adapt to force qemu-system-x86_64
to present a 32-bit system.

What about if we had qemu-system-i386 be a hardlink to
qemu-system-x86_64, and then changed behaviour based off the
executed binary name ?

ie if running qemu-system-i386, we could present a 32-bit CPU by
default. We eliminate all of our double compilation burden still.
We still have extra testing burden, but it is in a fairly narrow
area, so does not imply x2 the testing burden just $small-percentage
extra testing ?  That would means apps/users would not need to change
at all, but we still get most of the win we're after on the
QEMU side 

Essentially #ifdef TARGET_X86_64  would be change  'if (is_64bit) {...}'
in a handful of places, with 'bool is_64bit' initialized in main() from
argv[0] ?

With regards,
Daniel
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