Re: [PATCH RESEND] x86/entry: Don't write to CR3 when restoring to kernel CR3

[Ingo Molnar]

* Brendan Jackman <jackmanb@xxxxxxxxxx> wrote:

> From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> 
> Skip resuming KERNEL pages since it is already KERNEL CR3
> 
> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Brendan Jackman <jackmanb@xxxxxxxxxx>
> ---
> 
> While staring at paranoid_exit I was confused about why we had this CR3
> write, avoiding it seems like a free optimisation. The original commit
> 21e94459110252 ("x86/mm: Optimize RESTORE_CR3") says "Most NMI/paranoid
> exceptions will not in fact change pagetables" but I didn't't understand
> what the "most" was referring to. I then discovered this patch on the
> mailing list, Andy said[1] that it looks correct so maybe now is the
> time to merge it?
> 
> Note there's another patch in [1] as well, the benefit of that one is
> not obvious to me though.
> 
> We've tested an equivalent patch in our internal kernel.
> 
> [1] https://lore.kernel.org/lkml/20200526043507.51977-3-laijs@xxxxxxxxxxxxxxxxx/
> -- >8 --
>  arch/x86/entry/calling.h | 13 ++++---------
>  1 file changed, 4 insertions(+), 9 deletions(-)

I concur that this is a good change, but it would be really nice to get an 
ack from Andy or Thomas as well.

Thanks,

	Ingo