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Re: Setting constant-time mode CPU flag
- To: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 6 Sep 2022 11:52:05 +0200
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- Cc: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>, Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Xen developer discussion <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 06 Sep 2022 09:52:10 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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.
Jan
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