From: Peter Zijlstra <peterz@infradead.org>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: x86@kernel.org, Jann Horn <jannh@google.com>,
Natalie Silvanovich <natashenka@google.com>,
Seth Jenkins <sethjenkins@google.com>,
Kees Cook <keescook@chromium.org>,
linux-kernel@vger.kernel.org
Subject: Re: x86/mm: Randomize per-cpu entry area
Date: Fri, 7 Oct 2022 17:14:55 +0200 [thread overview]
Message-ID: <Y0BCb7mkLv75BqYK@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <Y0A7ZAjtDcq2PJaX@zx2c4.com>
On Fri, Oct 07, 2022 at 08:44:52AM -0600, Jason A. Donenfeld wrote:
> On Fri, Oct 07, 2022 at 10:42:36AM +0200, Peter Zijlstra wrote:
> > + max_cea = (CPU_ENTRY_AREA_MAP_SIZE - PAGE_SIZE) / CPU_ENTRY_AREA_SIZE;
> > +
> > + /* O(sodding terrible) */
> > + for_each_possible_cpu(i) {
> > + unsigned int cea;
> > +
> > +again:
> > + cea = prandom_u32_max(max_cea);
>
> Just FYI, max_cea is 2274876 (at least in my test rig), which means the
Yeah, that sounds about right, just over 21 bits.
> values returned from prandom_u32_max() won't be uniformly distributed.
> Right now the kernel doesn't have a function that does rejection
> sampling to get uniform distribution, but I could add one if you need.
I figured it was better than '% max_cea'.
> Alternatively, maybe you don't actually *need* this to be perfectly
> distributed -- which seems to be a common perspective -- and so this is
> good enough. Your call, but just wanted to make you aware.
First iteration had a LFSR to 'uniformly' and uniquely distribute the
CEAs around, but someone took offence to that :-). Anyway, I think
anything non-obvious is good enough in this case.
prev parent reply other threads:[~2022-10-07 15:15 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-07 8:42 x86/mm: Randomize per-cpu entry area Peter Zijlstra
2022-10-07 14:44 ` Jason A. Donenfeld
2022-10-07 15:14 ` Peter Zijlstra [this message]
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