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Re: [PATCH 4/4] pc-bios/s390-ccw: Allow building with Clang, too

From: David Hildenbrand
Subject: Re: [PATCH 4/4] pc-bios/s390-ccw: Allow building with Clang, too
Date: Mon, 3 May 2021 10:10:08 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1

On 03.05.21 07:17, Thomas Huth wrote:
On 03/05/2021 06.58, Markus Armbruster wrote:
Thomas Huth <thuth@redhat.com> writes:

Clang unfortunately does not support generating code for the z900
architecture level and starts with the z10 instead. Thus to be able
to support compiling with Clang, we have to check for the supported
compiler flags. The disadvantage is of course that the bios image
will only run with z10 guest CPUs upwards (which is what most people
use anyway), so just in case let's also emit a warning in that case.

What happens when you try to use this bios with an old CPU anyway?

Interesting question. I was expecting the guest to crash since it would be
using a CPU instruction that is not supported on the old CPU model. But I
just gave it a try, and there was no crash. The guest booted just fine.
Either Clang only emits instructions that work with the old z900 anyway, or
our emulation in QEMU is imprecise and we allow newer instructions to be
executed on old models, too.

Yes, that's currently still done. We once thought about disabling that (there was a patch from Richard), but decided against it because -- back then -- the default QEMU model was still very basic and would have essentially disabled all more recent instructions as default.

We can most probably do that change soon as we have a "fairly new" default QEMU CPU model. I can glue it to my z14 change.

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