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Re: Compiling the s390-ccw bios with clang (was: Re: s390-ccw: warning:

From: Thomas Huth
Subject: Re: Compiling the s390-ccw bios with clang (was: Re: s390-ccw: warning: writing 1 byte into a region of size 0)
Date: Fri, 23 Apr 2021 10:52:35 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.9.1

On 23/04/2021 10.47, Daniel P. Berrangé wrote:
On Fri, Apr 23, 2021 at 10:22:28AM +0200, Thomas Huth wrote:
On 23/04/2021 10.07, Cornelia Huck wrote:
On Fri, 23 Apr 2021 09:57:08 +0200
Thomas Huth <thuth@redhat.com> wrote:

On 23/04/2021 08.52, Christian Borntraeger wrote:
We can of course discuss if we compile the BIOS for z10 instead of z900. TCG
in the mean time can handle up to z13 and z10 is now also 13 years old.

I'd really like to see us supporting Clang in the s390-ccw bios, too, since
it provides additional useful compiler warnings ... but switching the -mz900
to -mz10 here also means that we could not boot VMs anymore that use a CPU
that is older than the z10...

We could still boot a kernel/initrd directly, couldn't we?

Yes, but that will certainly require some documentation effort to make it
clear to the users that they need to use "-kernel" in case they want to run
an older guest...

Is anybody still using such old CPUs? Should we maybe deprecate all CPUs
that are older than the z10 in QEMU? Alternatively, we could try to detect
Clang in the Makefile, and only use -mz10 in that case and continue to use
-mz900 in the other case...?

So, the issue with clang is that it compiles to at least a z10, right?

Right, Clang does not support anything that is older than a z10.

IIUC, according to wikipedia

    - z10 series was introduced in 2008
    - z900 series was introduced in 2000

Even the z10 is well older than the oldest OS platform we support.

Though I presume people keep mainframes deployed for longer than commodity
x86 hardware, it doesn't seem too unreasonable to say z10 is the oldest
we'll support.
Well, we're talking about *guest* support here. So with that argumentation, we could also remove support for old CPUs like "486" or "SandyBridge" from qemu-system-x86_64 ... should we maybe also start the deprecation there?


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