Re: [PATCH v2 09/25] KVM: VMX: Switch FRED RSP0 between host and guest

[Sean Christopherson]

On Fri, Jul 12, 2024, Xin3 Li wrote:
> > > > E.g. if it's somewhere in task_struct, then kvm_on_user_return()
> > > > would restore the current task's desired RSP0.
> > >
> > > So you're suggesting to extend the framework to allow per task constants?
> > 
> > Yeah, or more likely, special case MSR_IA32_FRED_RSP0.  If KVM didn't already
> > have the user return framework, I wouldn't suggest this as I doubt avoiding
> > WRMSR when switching between vCPU tasks will be very meaningful, but it's
> > easy to handle FRED_RSP0, so why not.
> 
> Great, I will take the patch.
> 
> It looks to me that this also works for KERNEL GS BASE MSR, no?

I don't think so, because the kernel expects MSR_KERNEL_GS_BASE to be accurate
when querying GS.base for the current task:

  unsigned long x86_gsbase_read_task(struct task_struct *task)
  {
	unsigned long gsbase;

	if (task == current)
		gsbase = x86_gsbase_read_cpu_inactive();
	else if (boot_cpu_has(X86_FEATURE_FSGSBASE) ||
		 (task->thread.gsindex == 0))
		gsbase = task->thread.gsbase;
	else
		gsbase = x86_fsgsbase_read_task(task, task->thread.gsindex);

	return gsbase;
  }