Re: [PATCH] x86/xstate: reset cached register values on resume

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 18/08/2021 12:30, Marek Marczykowski-Górecki wrote:
> set_xcr0() and set_msr_xss() use cached value to avoid setting the
> register to the same value over and over. But suspend/resume implicitly
> reset the registers and since percpu areas are not deallocated on
> suspend anymore, the cache gets stale.
> Reset the cache on resume, to ensure the next write will really hit the
> hardware. Choose value 0, as it will never be a legitimate write to
> those registers - and so, will force write (and cache update).
>
> Note the cache is used io get_xcr0() and get_msr_xss() too, but:
> - set_xcr0() is called few lines below in xstate_init(), so it will
>   update the cache with appropriate value
> - get_msr_xss() is not used anywhere - and thus not before any
>   set_msr_xss() that will fill the cache
>
> Fixes: aca2a985a55a "xen: don't free percpu areas during suspend"
> Signed-off-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>

I'd prefer to do this differently.  As I said in the thread, there are
other registers such as MSR_TSC_AUX which fall into the same category,
and I'd like to make something which works systematically.

~Andrew