x86/mm: Remove _PAGE_DIRTY from kernel RO pages

New processors that support Shadow Stack regard Write=0,Dirty=1 PTEs as
shadow stack pages.

In normal cases, it can be helpful to create Write=1 PTEs as also Dirty=1
if HW dirty tracking is not needed, because if the Dirty bit is not already
set the CPU has to set Dirty=1 when the memory gets written to. This
creates additional work for the CPU. So traditional wisdom was to simply
set the Dirty bit whenever you didn't care about it. However, it was never
really very helpful for read-only kernel memory.

When CR4.CET=1 and IA32_S_CET.SH_STK_EN=1, some instructions can write to
such supervisor memory. The kernel does not set IA32_S_CET.SH_STK_EN, so
avoiding kernel Write=0,Dirty=1 memory is not strictly needed for any
functional reason. But having Write=0,Dirty=1 kernel memory doesn't have
any functional benefit either, so to reduce ambiguity between shadow stack
and regular Write=0 pages, remove Dirty=1 from any kernel Write=0 PTEs.

Co-developed-by: Yu-cheng Yu <[email protected]>
Signed-off-by: Yu-cheng Yu <[email protected]>
Signed-off-by: Rick Edgecombe <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Reviewed-by: Borislav Petkov (AMD) <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Acked-by: Mike Rapoport (IBM) <[email protected]>
Tested-by: Pengfei Xu <[email protected]>
Tested-by: John Allen <[email protected]>
Tested-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/all/20230613001108.3040476-14-rick.p.edgecombe%40intel.com

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