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

[Thomas Huth <thuth@xxxxxxxxxx>]

On 06/03/2023 10.27, Daniel P. Berrangé wrote:
> On Mon, Mar 06, 2023 at 09:46:55AM +0100, Thomas Huth wrote:
> > Aside from not supporting KVM on 32-bit hosts, the qemu-system-x86_64
> > binary is a proper superset of the qemu-system-i386 binary. With the
> > 32-bit host support being deprecated, it is now also possible to
> > deprecate the qemu-system-i386 binary.
> >
> > With regards to 32-bit KVM support in the x86 Linux kernel,
> > the developers confirmed that they do not need a recent
> > qemu-system-i386 binary here:
> >
> >   https://lore.kernel.org/kvm/Y%2ffkTs5ajFy0hP1U@xxxxxxxxxx/
> >
> > Reviewed-by: Daniel P. Berrangé <berrange@xxxxxxxxxx>
> > Reviewed-by: Wilfred Mallawa <wilfred.mallawa@xxxxxxx>
> > Signed-off-by: Thomas Huth <thuth@xxxxxxxxxx>
> > ---
> >   docs/about/deprecated.rst | 14 ++++++++++++++
> >   1 file changed, 14 insertions(+)
> >
> > diff --git a/docs/about/deprecated.rst b/docs/about/deprecated.rst
> > index 1ca9dc33d6..c4fcc6b33c 100644
> > --- a/docs/about/deprecated.rst
> > +++ b/docs/about/deprecated.rst
> > @@ -34,6 +34,20 @@ deprecating the build option and no longer defend it in CI. 
> > The
> >   ``--enable-gcov`` build option remains for analysis test case
> >   coverage.
> >   
> > +``qemu-system-i386`` binary (since 8.0)
> > +'''''''''''''''''''''''''''''''''''''''
> > +
> > +The ``qemu-system-i386`` binary was mainly useful for running with KVM
> > +on 32-bit x86 hosts, but most Linux distributions already removed their
> > +support for 32-bit x86 kernels, so hardly anybody still needs this. The
> > +``qemu-system-x86_64`` binary is a proper superset and can be used to
> > +run 32-bit guests by selecting a 32-bit CPU model, including KVM support
> > +on x86_64 hosts. Thus users are recommended to reconfigure their systems
> > +to use the ``qemu-system-x86_64`` binary instead. 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,
> because we have code that changes the family/model/stepping for
> 'max' which is target dependent:
>
> #ifdef TARGET_X86_64
>      object_property_set_int(OBJECT(cpu), "family", 15, &error_abort);
>      object_property_set_int(OBJECT(cpu), "model", 107, &error_abort);
>      object_property_set_int(OBJECT(cpu), "stepping", 1, &error_abort);
> #else
>      object_property_set_int(OBJECT(cpu), "family", 6, &error_abort);
>      object_property_set_int(OBJECT(cpu), "model", 6, &error_abort);
>      object_property_set_int(OBJECT(cpu), "stepping", 3, &error_abort);
> #endif
>
> The former is a 64-bit AMD model and the latter is a 32-bit model.
>
> Seems LLVM was sensitive to this distinction to some extent:
>
>     https://gitlab.com/qemu-project/qemu/-/issues/191
>
> 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.
Ugh, ok. I gave it a quick try with a patch like this:

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -4344,15 +4344,15 @@ static void max_x86_cpu_initfn(Object *obj)
      */
     object_property_set_str(OBJECT(cpu), "vendor", CPUID_VENDOR_AMD,
                             &error_abort);
-#ifdef TARGET_X86_64
-    object_property_set_int(OBJECT(cpu), "family", 15, &error_abort);
-    object_property_set_int(OBJECT(cpu), "model", 107, &error_abort);
-    object_property_set_int(OBJECT(cpu), "stepping", 1, &error_abort);
-#else
-    object_property_set_int(OBJECT(cpu), "family", 6, &error_abort);
-    object_property_set_int(OBJECT(cpu), "model", 6, &error_abort);
-    object_property_set_int(OBJECT(cpu), "stepping", 3, &error_abort);
-#endif
+    if (object_property_get_bool(obj, "lm", &error_abort)) {
+        object_property_set_int(obj, "family", 15, &error_abort);
+        object_property_set_int(obj, "model", 107, &error_abort);
+        object_property_set_int(obj, "stepping", 1, &error_abort);
+    } else {
+        object_property_set_int(obj, "family", 6, &error_abort);
+        object_property_set_int(obj, "model", 6, &error_abort);
+        object_property_set_int(obj, "stepping", 3, &error_abort);
+    }
     object_property_set_str(OBJECT(cpu), "model-id",
                             "QEMU TCG CPU version " QEMU_HW_VERSION,
                             &error_abort);

... but it seems like the "lm" property is not initialized
there yet, so this does not work... :-/

Giving that we have soft-freeze tomorrow, let's ignore this patch
for now and revisit this topic during the 8.1 cycle. But I'll
queue the other 4 patches to get some pressure out of our CI
during the freeze time.

 Thomas