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

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Date:   Mon, 8 Aug 2022 17:35:16 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Dave Hansen <dave.hansen@...el.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        "H. Peter Anvin" <hpa@...or.com>,
        Michael Roth <michael.roth@....com>,
        Joerg Roedel <jroedel@...e.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 1/2] x86/sev: Put PSC struct on the stack in prep for
 unaccepted memory support

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