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Re: Setting constant-time mode CPU flag
- To: Jan Beulich <jbeulich@xxxxxxxx>, Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- Date: Tue, 6 Sep 2022 10:01:00 +0000
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- Cc: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>, Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>, Xen developer discussion <xen-devel@xxxxxxxxxxxxxxxxxxxx>
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- Thread-topic: Setting constant-time mode CPU flag
On 06/09/2022 10:52, Jan Beulich wrote:
> On 02.09.2022 04:05, Demi Marie Obenour wrote:
>> On Intel chips (Ice Lake and later) and ARM64, a bit needs to be set in
>> a CPU register to enforce constant-time execution. Linux plans to set
>> this bit by default; Xen should do the same. See
>> https://lore.kernel.org/lkml/YwgCrqutxmX0W72r@xxxxxxxxx/T/ for details.
>> I recommend setting the bit unconditionally and ignoring guest attempts
>> to change it.
> I don't think we ought to set it by default; I can see reasons why kernels
> may want to set it by default (providing a way to turn it off). In Xen
> what I think we need is exposure of the bit to be guest-controllable.
We absolutely should not have it set by default. It's a substantial
overhead for something that is only applicable to code which otherwise
crafted to be constant-time.
As for why Xen doesn't enumerate/virtualise it, that's because
virtualising MSR_ARCH_CAPS for guests is still not working yet, so the
feature can't be enumerated yet even if we did support context switching it.
~Andrew
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