Re: Emulating CPUs with larger atomic accesses

[Florian Weimer <fweimer@redhat.com>]

* Richard Henderson:

> On 5/24/22 02:27, Florian Weimer wrote:
>> * Richard Henderson:
>> 
>>> On 5/13/22 03:00, Florian Weimer wrote:
>>>> What's QEMU's approach to emulating CPU instructions that atomatically
>>>> operate on values larger than what is supported by the host CPU?
>>>> I assume that for full system emulation, this is not a problem, but
>>>> qemu-user will not achieve atomic behavior on shared memory mappings.
>>>> How much of a problem is this in practice?
>>>
>>> Well, it doesn't work, no.  In practice, x86_64 supports 128-bit
>>> atomic operations, and guest requires more than that.  No one really
>>> cares anymore about 32-bit hosts with smaller atomic operations.
>> Which part doesn't work?  Full-system emulation?
>
> No, user-only.
>
>> Do guests really require wider-than-128 atomics?  That's quite
>> surprising?
>
> Typo there -- "and no guest requires...".

Okay, thanks.  So the overall situation is okay even if we end up with
x86 guests that require CPU support for 128-bit loads.

Florian