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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Message-ID: <bff96726-cbf8-7f27-8f11-5bc979a8b407@amd.com>
Date:   Sun, 14 Aug 2022 08:36:47 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        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 8/13/22 14:40, Borislav Petkov wrote:
> On Fri, Aug 12, 2022 at 09:51:41AM -0500, Tom Lendacky wrote:
>> On 8/12/22 09:33, Borislav Petkov wrote:
>>> On Fri, Aug 12, 2022 at 09:11:25AM -0500, Tom Lendacky wrote:
>>>> There was a whole discussion on this
>>>
>>> Pointer to it?
>>
>> It starts here: https://lore.kernel.org/lkml/658c455c40e8950cb046dd885dd19dc1c52d060a.1659103274.git.thomas.lendacky@amd.com/
> 
> So how come none of the rationale for the on-stack decision vs a single
> buffer with a spinlock protection hasn't made it to this patch?
> 
> We need to have the reason why this thing is changed documented
> somewhere.

Yup, was all being addressed in v3 based on Dave's comments.

> 
>>> So smaller, on-stack PSC but lockless is still better than a bigger one
>>> but with synchronized accesses to it?
> 
> That thing.
> 
> That decision for on-stack buffer needs explaining why.
> 
>>>> Well when we don't know which GHCB is in use, using that reserved area in
>>>> the GHCB doesn't help.
>>>
>>> What do you mean?
>>>
>>> The one which you read with
>>>
>>> 	data = this_cpu_read(runtime_data);
>>
>> Memory acceptance is called before the per-CPU GHCBs have been allocated
>> and so you would be actually be using early boot GHCB. And that is decided
>> based on the #VC handler that is invoked - but in this case we're not
>> coming through the #VC handler to accept memory.
> 
> But then ghcb_percpu_ready needs to be a per-CPU variable too! Because
> it is set right after snp_register_per_cpu_ghcb() which works on the
> *per-CPU* GHCB.

No, and the code comment will explain this. Since the APs only ever use 
the per-CPU GHCB there is no concern as to when there is a switch over 
from the early boot GHCB to the per-CPU GHCB, so a single global variable 
is all that is needed.

I'll send out v3 soon.

Thanks,
Tom

>