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:   Fri, 12 Aug 2022 16:33:09 +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 09:11:25AM -0500, Tom Lendacky wrote:
> There was a whole discussion on this

Pointer to it?

> and I would prefer to keep the ability to parallelize PSC without
> locking.

So smaller, on-stack PSC but lockless is still better than a bigger one
but with synchronized accesses to it?

> 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);

in snp_register_per_cpu_ghcb() is the one you register.

> Also, I don't want to update the GHCB specification for a single bit
> that is only required because of the way Linux went about establishing
> the GHCB usage.

Linux?

You mean, you did it this way: 885689e47dfa1499b756a07237eb645234d93cf9

:-)

"The runtime handler needs one GHCB per-CPU. Set them up and map them
unencrypted."

Why does that handler need one GHCB per CPU?

As to the field, I was thinking along the lines of

	struct ghcb.vendor_flags

field which each virt vendor can use however they like.

It might be overkill but a random bool ain't pretty either. Especially
if those things start getting added for all kinds of other things.

If anything, you could make this a single u64 sev_flags which can at
least collect all that gunk in one variable ... at least...

Thx.

-- 
Regards/Gruss,
    Boris.

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