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

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@google.com/ > > > > Reviewed-by: Daniel P. Berrangé <berrange(a)redhat.com&gt; > > Reviewed-by: Wilfred Mallawa <wilfred.mallawa(a)wdc.com&gt; > > Signed-off-by: Thomas Huth <thuth(a)redhat.com&gt; > > --- > > 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.