Re: [RFC PATCH] docs/about/deprecated: Deprecate 32-bit host systems

[Daniel P . Berrangé]

On Wed, Feb 22, 2023 at 09:11:13AM +0000, Bernhard Beschow wrote:
> 
> 
> Am 30. Januar 2023 20:45:47 UTC schrieb "Alex Bennée" 
> <alex.bennee@linaro.org>:
> >
> >Daniel P. Berrangé <berrange@redhat.com> writes:
> >
> >> On Mon, Jan 30, 2023 at 11:47:02AM +0000, Peter Maydell wrote:
> >>> On Mon, 30 Jan 2023 at 11:44, Thomas Huth <thuth@redhat.com> wrote:
> >>> >
> >>> > Testing 32-bit host OS support takes a lot of precious time during the 
> >>> > QEMU
> >>> > contiguous integration tests, and considering that many OS vendors 
> >>> > stopped
> >>> > shipping 32-bit variants of their OS distributions and most hardware 
> >>> > from
> >>> > the past >10 years is capable of 64-bit
> >>> 
> >>> True for x86, not necessarily true for other architectures.
> >>> Are you proposing to deprecate x86 32-bit, or all 32-bit?
> >>> I'm not entirely sure about whether we're yet at a point where
> >>> I'd want to deprecate-and-drop 32-bit arm host support.
> >>
> >> Do we have a feeling on which aspects of 32-bit cause us the support
> >> burden ? The boring stuff like compiler errors from mismatched integer
> >> sizes is mostly quick & easy to detect simply through a cross compile.
> >>
> >> I vaguely recall someone mentioned problems with atomic ops in the past,
> >> or was it 128-bit ints, caused implications for the codebase ?
> >
> >Atomic operations on > TARGET_BIT_SIZE and cputlb when
> >TCG_OVERSIZED_GUEST is set. Also the core TCG code and a bunch of the
> >backends have TARGET_LONG_BITS > TCG_TARGET_REG_BITS ifdefs peppered
> >throughout.
> 
> Are there any plans or ideas to support 128 bit architectures
> such as CHERI in the future? There is already a QEMU fork
> implementing CHERI for RISC V [1]. Also ARM has developed an
> experimental hardware implementation of CHERI within the Morello
> project where Linaro is involved as well, although the QEMU
> implementation is performed by the University of Cambridge [2].

If 128 bit hardware exists and has real world non-toy usage,
then a request to support it in QEMU is essentially inevitable.

> I'm asking because once we deeply bake in the assumption that
> host size >= guest size then adding such architectures will
> become much harder.

Yep, that is a risk.

With regards,
Daniel
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