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Re: [PATCH 0/2] Change default pointer authentication algorithm on aarch


From: Pierrick Bouvier
Subject: Re: [PATCH 0/2] Change default pointer authentication algorithm on aarch64 to impdef
Date: Mon, 16 Dec 2024 11:26:47 -0800
User-agent: Mozilla Thunderbird

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.

And I'm not sure we want to introduce the notion of "versioned" cpu just for this. Maybe we could simply restrict migration between major QEMU versions in this scenario.


r~




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