* Emulating CPUs with larger atomic accesses
@ 2022-05-13 10:00 Florian Weimer
2022-05-22 1:07 ` Richard Henderson
0 siblings, 1 reply; 5+ messages in thread
From: Florian Weimer @ 2022-05-13 10:00 UTC (permalink / raw)
To: qemu-devel
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?
Thanks,
Florian
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Emulating CPUs with larger atomic accesses
2022-05-13 10:00 Emulating CPUs with larger atomic accesses Florian Weimer
@ 2022-05-22 1:07 ` Richard Henderson
2022-05-24 9:27 ` Florian Weimer
0 siblings, 1 reply; 5+ messages in thread
From: Richard Henderson @ 2022-05-22 1:07 UTC (permalink / raw)
To: Florian Weimer, qemu-devel
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.
r~
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Emulating CPUs with larger atomic accesses
2022-05-22 1:07 ` Richard Henderson
@ 2022-05-24 9:27 ` Florian Weimer
2022-05-24 11:48 ` Richard Henderson
0 siblings, 1 reply; 5+ messages in thread
From: Florian Weimer @ 2022-05-24 9:27 UTC (permalink / raw)
To: Richard Henderson; +Cc: qemu-devel
* 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?
Do guests really require wider-than-128 atomics? That's quite
surprising?
Thanks,
Florian
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Emulating CPUs with larger atomic accesses
2022-05-24 9:27 ` Florian Weimer
@ 2022-05-24 11:48 ` Richard Henderson
2022-05-24 11:51 ` Florian Weimer
0 siblings, 1 reply; 5+ messages in thread
From: Richard Henderson @ 2022-05-24 11:48 UTC (permalink / raw)
To: Florian Weimer; +Cc: qemu-devel
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...".
r~
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Emulating CPUs with larger atomic accesses
2022-05-24 11:48 ` Richard Henderson
@ 2022-05-24 11:51 ` Florian Weimer
0 siblings, 0 replies; 5+ messages in thread
From: Florian Weimer @ 2022-05-24 11:51 UTC (permalink / raw)
To: Richard Henderson; +Cc: qemu-devel
* 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
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2022-05-24 12:25 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz follow: Atom feed
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2022-05-13 10:00 Emulating CPUs with larger atomic accesses Florian Weimer
2022-05-22 1:07 ` Richard Henderson
2022-05-24 9:27 ` Florian Weimer
2022-05-24 11:48 ` Richard Henderson
2022-05-24 11:51 ` Florian Weimer
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