> > > > Switch MSR_IA32_FRED_RSP0 between host and guest in
> > > > vmx_prepare_switch_to_{host,guest}().
> > > >
> > > > MSR_IA32_FRED_RSP0 is used during ring 3 event delivery only, thus
> > > > KVM, running on ring 0, can run safely with guest FRED RSP0, i.e., no
> > > > need to switch between host/guest FRED RSP0 during VM entry and exit.
> > > >
> > > > KVM should switch to host FRED RSP0 before returning to user level,
> > > > and switch to guest FRED RSP0 before entering guest mode.
> > >
> > > Heh, if only KVM had a framework that was specifically designed for context
> > > switching MSRs on return to userspace. Translation: please use the
> > > user_return_msr() APIs.
> >
> > IIUC the user return MSR framework works for MSRs that are per CPU
> > constants, but like MSR_KERNEL_GS_BASE, MSR_IA32_FRED_RSP0 is a per
> > *task* constant, thus we can't use it.
>
> Ah, in that case, the changelog is very misleading and needs to be fixed.
I probably should've given more details about how FRED RSPs are used:
RSP0 is a bit of special because it's a per task constant pointing to
its kernel stack top, while other RSPs are per CPU constants.
> Alternatively, is the desired RSP0 value tracked anywhere other than the MSR?
Yes, It's simply "(unsigned long)task_stack_page(task) + THREAD_SIZE".
> 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?
> Even if we don't get fancy, avoiding the RDMSR
> to get the current task's value would be nice.
TBH, I didn't know RDMSR is NOT a preferred approach. But it does make
sense because it costs hundreds cycles to read from CR2.
And of course this can be easily changed to
"(unsigned long)task_stack_page(task) + THREAD_SIZE".
