Re: [PATCH 0/2] Change default pointer authentication algorithm on aarch64 to impdef

[Alex Bennée]

Pierrick Bouvier <pierrick.bouvier@linaro.org> writes:

> On 12/16/24 11:50, Richard Henderson wrote:
>> On 12/16/24 13:26, Pierrick Bouvier wrote:
>>> On 12/16/24 11:10, Richard Henderson wrote:
>>>> On 12/4/24 15:12, Pierrick Bouvier wrote:
>>>>> qemu-system-aarch64 default pointer authentication (QARMA5) is expensive, 
>>>>> we
>>>>> spent up to 50% of the emulation time running it (when using TCG).
>>>>>
>>>>> Switching to pauth-impdef=on is often given as a solution to speed up 
>>>>> execution.
>>>>> Thus we talked about making it the new default.
>>>>>
>>>>> The first patch introduce a new property (pauth-qarma5) to allow to select
>>>>> current default algorithm.
>>>>> The second one change the default.
>>>>>
>>>>> Pierrick Bouvier (2):
>>>>>      target/arm: add new property to select pauth-qarma5
>>>>>      target/arm: change default pauth algorithm to impdef
>>>>>
>>>>>     docs/system/arm/cpu-features.rst |  7 +++++--
>>>>>     docs/system/introduction.rst     |  2 +-
>>>>>     target/arm/cpu.h                 |  1 +
>>>>>     target/arm/arm-qmp-cmds.c        |  2 +-
>>>>>     target/arm/cpu64.c               | 30 +++++++++++++++++++-----------
>>>>>     tests/qtest/arm-cpu-features.c   | 15 +++++++++++----
>>>>>     6 files changed, 38 insertions(+), 19 deletions(-)
>>>>>
>>>>
>>>> I understand the motivation, but as-is this will break migration.
>>>>
>>>> I think this will need to be versioned somehow, but the only thing that 
>>>> really gets
>>>> versioned are the boards, and I'm not sure how to link that to the 
>>>> instantiated cpu.
>>>>
>>>
>>>   From what I understood, and I may be wrong, the use case to migrate (tcg) 
>>> vm with cpu max
>>> between QEMU versions is *not* supported, as we can't guarantee which 
>>> features are present
>>> or not.
>> This doesn't affect only -cpu max, but anything using 
>> aarch64_add_pauth_properties():
>> neoverse-n1, neoverse-n2, cortex-a710.
>> 
>
> I think this is still a change worth to do, because people can get a
> 100% speedup with this simple change, and it's a better default than
> the previous value.
> In more, in case of this migration scenario, QEMU will immediately
> abort upon accessing memory through a pointer.
>
> I'm not sure about what would be the best way to make this change as
> smooth as possible for QEMU users.

Surely we can only honour and apply the new default to -cpu max?

>
> Peter, Alex, do you have any suggestion on this topic?
>
> Thanks,
> Pierrick
>
>> r~

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro