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Re: Emulating CPUs with larger atomic accesses
From: |
Florian Weimer |
Subject: |
Re: Emulating CPUs with larger atomic accesses |
Date: |
Tue, 24 May 2022 13:51:00 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/27.2 (gnu/linux) |
* 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