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:   Sat, 13 Aug 2022 21:40:14 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Tom Lendacky <thomas.lendacky@....com>
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 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.

> > 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.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette