Re: [PATCH v30 07/20] x86/sgx: Enumerate and track EPC sections

From: Borislav Petkov <bp@alien8.de>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
	linux-kernel@vger.kernel.org, x86@kernel.org,
	linux-sgx@vger.kernel.org, akpm@linux-foundation.org,
	dave.hansen@intel.com, nhorman@redhat.com, npmccallum@redhat.com,
	haitao.huang@intel.com, andriy.shevchenko@linux.intel.com,
	tglx@linutronix.de, kai.svahn@intel.com, josh@joshtriplett.org,
	luto@kernel.org, kai.huang@intel.com, rientjes@google.com,
	cedric.xing@intel.com, puiterwijk@redhat.com,
	Serge Ayoun <serge.ayoun@intel.com>,
	Jethro Beekman <jethro@fortanix.com>
Subject: Re: [PATCH v30 07/20] x86/sgx: Enumerate and track EPC sections
Date: Wed, 27 May 2020 22:35:09 +0200	[thread overview]
Message-ID: <20200527203509.GF1721@zn.tnic> (raw)
In-Reply-To: <20200527035613.GH31696@linux.intel.com>

On Tue, May 26, 2020 at 08:56:14PM -0700, Sean Christopherson wrote:
> > > +		if (i == ARRAY_SIZE(sgx_epc_sections)) {
> > > +			pr_warn("No free slot for an EPC section\n");
> > > +			break;
> > > +		}
> > 
> > This is also the loop termination: do we really need this warn or can
> > the loop simply do "i < ARRAY_SIZE" ?
> 
> The warn alerts the user that there will effectively be lost/unused memory,
> so IMO it's worth keeping.

Can the user do anything about it? If not, then probably no need to say
anything.

> > > +		pr_info("EPC section 0x%llx-0x%llx\n", pa, pa + size - 1);
> > 
> > I'm assuming that's useful information to issue in dmesg?
> 
> Yes, it's effectively the equivalent of dumping the e820 tables.  It might
> not be as useful now that the code is stable, but I suspect it will come in
> handy for debug/triage down the road.

Ok.

> > Same for this one: also shared only among sgx code?
> 
> Yes, that one can definitely be buried behind a helper.

Shared only within sgx is fine. 

<snip> 

> One idea would be to provide a Kconfig a la NR_CPUS or NODES_SHIFT.  I.e.
> carve out the bits in sgx_epc_page_desc to allow up to N sections, but let
> the user limit the number of sections to recoup the unused memory.

I'd prefer a good estimate of a highest value, say 8 bits, which should
suffice for the foreseeable future. That's the simplest thing to do and
our Kconfig symbols space is an abomination of gazillion symbols.

Thx.

-- 
Regards/Gruss,
    Boris.

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