Re: [PATCH v2 1/2] x86/sev: Put PSC struct on the stack in prep for unaccepted memory support

[Tom Lendacky]

On 8/8/22 17:33, Dave Hansen wrote:
> On 8/8/22 15:18, Tom Lendacky wrote:
> > > >    +/* Flag to indicate when the first per-CPU GHCB is registered */
> > > > +static bool ghcb_percpu_ready __section(".data");
> > >
> > > So, there's a code path that can't be entered until this is set?  Seems
> > > like the least we can do it annotate that path with a
> > > WARN_ON_ONCE(!ghcb_percpu_ready).
> >
> > Sure, that can be added. Right now the only function that calls
> > vmgexit_psc() is covered (set_pages_state()/__set_pages_state()) and is
> > doing the right thing. But I guess if anything is added in the future,
> > that will provide details on what happened.
> >
> > > Also, how does having _one_ global variable work for indicating the
> > > state of multiple per-cpu structures?  The code doesn't seem to delay
> > > setting this variable until after _all_ of the per-cpu state is ready.
> >
> > All of the per-CPU GHCBs are allocated during the BSP boot, before any
> > AP is started. The APs only ever run the kernel_exc_vmm_communication()
> > #VC handler and only ever use the per-CPU version of the GHCB, never the
> > early boot version. This is based on the initial_vc_handler being
> > switched to the runtime #VC handler, kernel_exc_vmm_communication.
> >
> > The trigger for the switch over for the BSP from the early boot GHCB to
> > the per-CPU GHCB is during setup_ghcb() after the initial_vc_handler has
> > been switched to kernel_exc_vmm_communication, which is just after the
> > per-CPU allocations. By putting the setting of the ghcb_percpu_ready in
> > setup_ghcb(), it indicates that the BSP per-CPU GHCB has been registered
> > and can be used.
>
> That description makes the proposed comment even more confusing:
>
> 	/* Flag to indicate when the first per-CPU GHCB is registered */
>
> The important thing is that this variable is only _useful_ for the boot
> CPU.  After the boot CPU has allocated space for _itself_, it can then
> go and stop using the MSR-based method.
>
> The reason it's set after "the first" is because "the first" is also the
> boot CPU, but referring to it as the "the first" is a bit oblique.
> Maybe something like this:
>
> 	/*
> 	 * Set after the boot CPU's GHCB is registered.  At that point,
> 	 * it can be used for calls instead of MSRs.
> 	 */

Sure, I'll work on the wording.

Thanks,
Tom