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

On Fri, 2022-02-11 at 15:37 -0800, Dave Hansen wrote:

On 1/30/22 13:18, Rick Edgecombe wrote:

quoted

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.