Re: [PATCH 25/35] x86/cet/shstk: Add user-mode shadow stack support

Edgecombe, Rick P
2022-02-12T00:12:16Z

On Fri, 2022-02-11 at 15:37 -0800, Dave Hansen wrote:
> On 1/30/22 13:18, Rick Edgecombe wrote:
> > Add the user shadow stack MSRs to the xsave helpers, so they can be
> > used
> > to implement the functionality.
>
> Do these MSRs ever affect kernel-mode operation?
>
> If so, we might need to switch them more aggressively at context-
> switch
> time like PKRU.
>
> If not, they can continue to be context-switched with the PASID state
> which does not affect kernel-mode operation.
>
> Either way, it would be nice to have some changelog material to that
> effect.

The only special shadow stack thing the kernel does is WRUSS, which per
the SDM only needs the CR4 bit set to work (unlike WRSS). So I think
the lazy restore is ok.