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

[Dave Hansen <dave.hansen@intel.com>]

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.
	 */