linux-kernel - Re: [PATCH v13 2/2] x86/sgx: Add an attribute for the amount of SGX memory in a NUMA node

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Date:   Tue, 7 Dec 2021 11:36:50 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     reinette.chatre@...el.com, linux-kernel@...r.kernel.org,
        linux-sgx@...r.kernel.org
Subject: Re: [PATCH v13 2/2] x86/sgx: Add an attribute for the amount of SGX
 memory in a NUMA node

On 11/16/21 8:21 AM, Jarkko Sakkinen wrote:
> The amount of SGX memory on the system is determined by the BIOS and it
> varies wildly between systems.  It can be from dozens of MB's on desktops
> or VM's, up to many GB's on servers.  Just like for regular memory, it is
> sometimes useful to know the amount of usable SGX memory in the system.
> 
> Introduce CONFIG_HAVE_ARCH_NODE_DEV_GROUP opt-in flag to expose an arch
> specific attribute group, and add an attribute for the amount of SGX
> memory in bytes to each NUMA node:
> 
> /sys/devices/system/node/nodeX/x86/sgx_total_bytes

There's some context missing here:

This serves the same function for SGX memory as /proc/meminfo or
/sys/devices/system/node/nodeX/meminfo does for normal RAM.  It
enumerates how much physical SGX memory is present so that you can size
enclaves on different systems.

This specific file (sgx_total_bytes) is needed today to help drive the
SGX selftests.  The SGX selftests need to create overcommitted enclaves
which are larger than the physical SGX memory on the system.  They
currently use a CPUID-based approach which can diverge from the actual
amount of SGX memory available.  This file ensures that the selftests
can work efficiently and do not attempt stupid things like creating a
100,000 MB enclave on a system with 128 MB  of SGX memory.

The nodeX/x86 directory is used because SGX is highly x86-specific.
It's very unlikely that any other architecture (or even non-Intel x86
vendor) will ever implement SGX.  It needs its own directory (as opposed
to being in the nodeX/ "root") because this is expected to be the first
of a few different things that need to get exported.  This avoids
cluttering the root with several "sgx_*" files.

How many of these files will there be?  Just scanning /proc/meminfo,
these are the no-brainers that we have for RAM, but we need for SGX:

MemTotal:       xxxx kB // sgx_total_bytes (this patch)
MemFree:        yyyy kB // sgx_free_bytes
SwapTotal:      zzzz kB // sgx_swapped_bytes

So, at *least* three.  I think we will eventually end up needing
something more along the lines of a dozen.