From: Linus Torvalds <torvalds@linux-foundation.org>
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>
Subject: Re: Fix built-in early-load Intel microcode alignment
Date: Wed, 15 Jan 2020 11:41:46 -0800 [thread overview]
Message-ID: <CAHk-=whOC9dakUZ_BzHq2d5oKXXGnrKf+M-4gZ8U+=F_OX4+Ew@mail.gmail.com> (raw)
In-Reply-To: <20200115185812.GH11244@42.do-not-panic.com>
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
next prev parent reply other threads:[~2020-01-15 19:42 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-12 13:00 Fix built-in early-load Intel microcode alignment Jari Ruusu
2020-01-12 13:03 ` Jari Ruusu
2020-01-12 14:02 ` Greg KH
2020-01-13 6:30 ` Jari Ruusu
2020-01-13 6:42 ` Greg KH
2020-01-13 15:47 ` Luis Chamberlain
2020-01-13 19:44 ` Linus Torvalds
2020-01-15 2:27 ` Luis Chamberlain
2020-01-18 20:10 ` Bjorn Andersson
2020-01-13 19:58 ` Jari Ruusu
2020-01-13 20:08 ` Borislav Petkov
2020-01-13 20:30 ` Jari Ruusu
2020-01-13 20:46 ` Borislav Petkov
2020-01-15 2:15 ` Luis Chamberlain
2020-01-15 18:46 ` Jari Ruusu
2020-01-15 18:58 ` Luis Chamberlain
2020-01-15 19:41 ` Linus Torvalds [this message]
2020-01-15 19:00 ` Andy Lutomirski
2020-01-15 19:15 ` Jari Ruusu
2020-01-15 19:49 ` Linus Torvalds
2020-01-16 6:55 ` Jari Ruusu
2020-01-16 19:16 ` Raj, Ashok
2020-01-17 9:47 ` Jari Ruusu
2020-02-03 20:10 ` Luis Chamberlain
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to='CAHk-=whOC9dakUZ_BzHq2d5oKXXGnrKf+M-4gZ8U+=F_OX4+Ew@mail.gmail.com' \
--to=torvalds@linux-foundation.org \
--cc=bp@alien8.de \
--cc=fenghua.yu@intel.com \
--cc=hdegoede@redhat.com \
--cc=jari.ruusu@gmail.com \
--cc=johannes.berg@intel.com \
--cc=linux-kernel@vger.kernel.org \
--cc=luto@kernel.org \
--cc=mcgrof@kernel.org \
--cc=stable@vger.kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).