Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <CACMCwJK-2DHZDA_F5Z3wsEUEKJSc3uOwwPD4HRoYGW7A+kA75w@mail.gmail.com>
 <20200113154739.GB11244@42.do-not-panic.com> <CACMCwJL8tu+GHPeRADR_12xhcYSiDv+Yxdy=yLqMxEsn=P9zFA@mail.gmail.com>
 <20200115021545.GD11244@42.do-not-panic.com> <CACMCwJLJCA2iXS0QMKKAWQv252oUcmfsNvwDNP5+4Z_9VB-rTg@mail.gmail.com>
 <20200115185812.GH11244@42.do-not-panic.com>
In-Reply-To: <20200115185812.GH11244@42.do-not-panic.com>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 15 Jan 2020 11:41:46 -0800
Message-ID: <CAHk-=whOC9dakUZ_BzHq2d5oKXXGnrKf+M-4gZ8U+=F_OX4+Ew@mail.gmail.com>
Subject: Re: Fix built-in early-load Intel microcode alignment
To:     Luis Chamberlain <mcgrof@kernel.org>
Cc:     Jari Ruusu <jari.ruusu@gmail.com>, Borislav Petkov <bp@alien8.de>,
        Fenghua Yu <fenghua.yu@intel.com>, johannes.berg@intel.com,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        stable <stable@vger.kernel.org>,
        Hans de Goede <hdegoede@redhat.com>,
        Andy Lutomirski <luto@kernel.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Jan 15, 2020 at 10:58 AM Luis Chamberlain <mcgrof@kernel.org> wrote:
>
> But *how? Why is there a 50/50 chance of it being aligned to
> 16 bytes if 8 bytes are currently specified?

What?

It's trivial.

Address 256 is 4-byte aligned. But it's also 8-byte aligned. And
16-byte aligned. And..

So if you ask for 8-byte alignment, and you already had that address
(or were just below it), you'll get 8-byte alignment. But it will
_also_ be 16-byte aligned just by happenstance.

And yes, exactly half of the addresses that are 8-byte aligned are
also 16-byte aligned, so you have a 50/50 chance of getting the bigger
alignment simply by random chance.

In fact, often you probably have a _better_ than 50/50 chance of
getting the bigger alignment, since many other things are aligned too,
and the starting address likely isn't very random. So it might have
started out with a bigger alignment even before you asked for just
8-byte aligned data from the linker.

(Of course, the reverse may be true too - there may be cases you were
coimpletely mis-aligned, and asking for 8-byte alignment will never
give you any more aligned memory, but I suspect aligned data is a lot
more common than unaligned data is)

              Linus