Re: [PATCH RFC V2 17/17] x86/entry: Preserve PKRS MSR across exceptions

Fenghua Yu
2020-07-23T16:52:09Z

Hi, Dave,

On Thu, Jul 23, 2020 at 09:23:13AM -0700, Dave Hansen wrote:
> On 7/23/20 9:18 AM, Fenghua Yu wrote:
> > The PKRS MSR has been preserved in thread_info during kernel entry. We
> > don't need to preserve it in another place (i.e. idtentry_state).
>
> I'm missing how the PKRS MSR gets preserved in thread_info. Could you
> explain the mechanism by which this happens and point to the code
> implementing it, please?

[Sorry, my mistake: I mean "thread_struct" instead of "thread_info".
Hopefully the typo doesn't change the essential part in my last email.]

The "saved_pkrs" is defined in thread_struct and context switched in
patch 04/17:
https://lore.kernel.org/lkml/20200717072056.73134-5-ira.weiny@intel.com/

Because there is no XSAVE support the PKRS MSR, we preserve it in
"saved_pkrs" in thread_struct. It's initialized as 0 (init state, no
protection key) in fork() or exec(). It's updated to a right protection
value when a driver calls the updating API. The PKRS MSR is context
switched by "saved_pkrs" when switching to a task (unless optimized if the
cached MSR is the same as the saved one).

Thanks.

-Fenghua