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

[Borislav Petkov <bp@alien8.de>]

On Tue, Dec 07, 2021 at 11:36:50AM -0800, Dave Hansen wrote:
> 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:

I'll say. "it is sometimes useful" is not the proper argumentation for
adding new stuff.

> 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.

Thanks, that's exactly what I was missing. Please stick that explanation
somewhere prominent.

> 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.

Oh well, I guess.

I was going to propose to make that

	...nodeX/sgx/

but having it be arch-specific would mean that theoretically we could
add some non-sgx files there too, in the future, and if/when needed.

Thx.

-- 
Regards/Gruss,
    Boris.

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