linux-kernel - Re: [PATCH v3 2/7] x86/sgx: Add infrastructure to identify SGX EPC pages

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Date:   Tue, 3 Aug 2021 23:49:35 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     "Hansen, Dave" <dave.hansen@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>
Subject: Re: [PATCH v3 2/7] x86/sgx: Add infrastructure to identify SGX EPC
 pages



Sent from my iPhone

> On Aug 3, 2021, at 14:47, Matthew Wilcox
> 
> Liam and I are working on a new replacement data structure called the
> Maple Tree, but it's not yet ready to replace the radix tree back end.
> It looks like it would be perfect for your case; there would be five
> entries in it, stored in one 256-byte node:
> 
>    NULL
> 0x8000bfffff
>    p1
> 0x807f7fffff
>    NULL
> 0x10000c00000
>    p2
> 0x1007fffffff
>    NULL
> 0xffff'ffff'ffff'ffff
> 
> It would actually turn into a linear scan, because that's just the
> fastest way to find something in a list of five elements.  A third
> range would take us to a list of seven elements, which still fits
> in a single node.  Once we get to more than that, you'd have a
> two-level tree, which would work until you have more than ~20 ranges.
> 
> We could do better for your case by storing 10x (start, end, p) in each
> leaf node, but we're (currently) optimising for VMAs which tend to be
> tightly packed, meaning that an implicit 'start' element is a better
> choice as it gives us 15x (end, p) pairs.

That’s good to know. While current xarray
implementation might be a bit wasteful[1],
things will get better.

I’m still going with xarray to keep the source
simple.

-Tony

[1] A few KBytes extra doesn’t even sound
too terrible to manage tens of MBytes (or
more) of SGX EPC memory on a system
with a half TByte total memory.