linux-kernel - Re: [PATCH 1/2] x86/sgx: Add accounting for tracking overcommit

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Date:   Tue, 21 Dec 2021 07:53:38 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Borislav Petkov <bp@...en8.de>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>
Cc:     linux-sgx@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] x86/sgx: Add accounting for tracking overcommit

On 12/20/21 2:48 PM, Borislav Petkov wrote:
> On Mon, Dec 20, 2021 at 01:35:03PM -0800, Kristen Carlson Accardi wrote:
>> So, in your proposal you would first change the calculated number of
>> maximum available backing pages to be based on total system RAM rather
>> than EPC memory, got it.
> 
> This was just an example. My point is to try to make it automatic and
> not introduce another knob. And to decide on the limits and behavior
> by using common sense and addressing the common use cases first.

The common case is clearly a few enclaves on systems where the
overcommit levels are modest.  The "100%" limit will work great there.

I'd personally be fine with just enforcing that limit as the default and
ignoring everything else.  It makes me a bit nervous, though, because
suddenly enforcing a limit is an ABI break.  The tunable effectively
gives us a way out if we screw up either the limit's quantity or someone
needs the old ABI back.

That said, we don't *need* it to be tunable, boot parameter or not.  If
you're concerned about the tunable, I think we should drop it and not
look back.

> Imagine you're a sysadmin. Or a general, common system user if there
> ever is one.
> 
> When your system starts thrashing and you're running a bunch of
> enclaves, how do you find out there even is a knob which might
> potentially help you?

I'm selfish.  The tunable isn't for end users.  It's for me.

The scenario I envisioned is that a user upgrades to a new kernel and
their enclaves start crashing.  They'll eventually find us, the
maintainers of the SGX code, and we'll have a tool as kernel developers
to help them.  The tunable helps _me_ in two ways:
1. It help me easily get user back to pre-5.17 (or whatever) behavior
2. If we got the "100%" value wrong, end users can help us experiment to
   help find a better value.

BTW, all the chat about "malicious" enclaves and so forth...  I
*totally* agree that this is a problem and one worth solving.  It just
can't be solved today.  We need real cgroup support.  It's coming soon.