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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Message-ID: <32abd0a7-bc3c-f7a7-3330-8287ef05131c@intel.com>
Date:   Wed, 8 Dec 2021 11:38:11 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Jarkko Sakkinen <jarkko@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        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>, 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

I reworked the changelog quite a bit, addressing some of Borislav's
questions.  No code changes, though.

The result is below.  I've retained Greg's ack.  I'll stick this in
tip/x86/sgx if this looks OK to everyone.

---

From: Jarkko Sakkinen <jarkko@...nel.org>

== Problem ==

The amount of SGX memory on a system is determined by the BIOS and it
varies wildly between systems.  It can be as small as dozens of MB's
and as large as many GB's on servers.  Just like how applications need
to know how much regular RAM is available, enclave builders need to
know how much SGX memory an enclave can consume.

== Solution ==

Introduce a new sysfs file:

	/sys/devices/system/node/nodeX/x86/sgx_total_bytes

to enumerate the amount of SGX memory available in each NUMA node.
This serves the same function for SGX as /proc/meminfo or
/sys/devices/system/node/nodeX/meminfo does for normal RAM.

'sgx_total_bytes' is needed today to help drive the SGX selftests.
SGX-specific swap code is exercised by creating 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.  'sgx_total_bytes' 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.

== Implementation Details ==

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:

== ABI Design Discussion ==

As opposed to the per-node ABI, a single, global ABI was considered.
However, this would prevent enclaves from being able to size
themselves so that they fit on a single NUMA node.  Essentially, a
single value would rule out NUMA optimizations for enclaves.

Create a new "x86/" directory inside each "nodeX/" sysfs directory.
'sgx_total_bytes' is expected to be the first of at least a few
sgx-specific files to be placed in the new directory.  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 (implemented here)
	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.  A new directory (as
opposed to being in the nodeX/ "root") directory avoids cluttering the
root with several "sgx_*" files.

Place the new file in a new "nodeX/x86/" directory because SGX is
highly x86-specific.  It is very unlikely that any other architecture
(or even non-Intel x86 vendor) will ever implement SGX.  Using "sgx/"
as opposed to "x86/" was also considered.  But, there is a real chance
this can get used for other arch-specific purposes.

[ dhansen: rewrite changelog ]

Signed-off-by: Jarkko Sakkinen <jarkko@...nel.org>
Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Acked-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Link: https://lkml.kernel.org/r/20211116162116.93081-2-jarkko@kernel.org
---
 Documentation/ABI/stable/sysfs-devices-node |  6 ++++++
 arch/Kconfig                                |  4 ++++
 arch/x86/Kconfig                            |  1 +
 arch/x86/kernel/cpu/sgx/main.c              | 20 ++++++++++++++++++++
 arch/x86/kernel/cpu/sgx/sgx.h               |  1 +
 drivers/base/node.c                         |  3 +++
 include/linux/numa.h                        |  4 ++++
 7 files changed, 39 insertions(+)