RE: [PATCH v5 23/34] x86/fred: update MSR_IA32_FRED_RSP0 during task switch

[Li, Xin3]

> > -	if (cpu_feature_enabled(X86_FEATURE_XENPV))
> > +	if (cpu_feature_enabled(X86_FEATURE_FRED)) {
> > +		/*
> > +		 * Will use WRMSRNS/WRMSRLIST for performance once it's
> upstreamed.
> > +		 */
> > +		wrmsrl(MSR_IA32_FRED_RSP0,
> > +		       task_top_of_stack(task) +
> TOP_OF_KERNEL_STACK_PADDING);
> > +	} else if (cpu_feature_enabled(X86_FEATURE_XENPV)) {
> 
> Whee, so hardware will really only ever look at this when RSP0? I don't need to
> worry about exceptions during context switch?

You're right, we don't.

RSP0 is only used in ring3. Exceptions from ring0 just keep using the current
kernel stack unless a higher stack level needs to be used, e.g., RSP3 for #DF.

Thanks!
  Xin