[U-Boot] [RFC] [PATCH] rewrite doc/README.arm-unaligned-accesses

Tom Rini
2014-02-20T21:34:44Z

On Tue, Feb 18, 2014 at 01:52:42PM +0100, Albert ARIBAUD wrote:

> There has been a few back-and-forths (and sideways too) about how
> unaligned accesses are considered in ARM U-Boot. This post is to (try
> and) get us together in one place, get things straight about what is
> currently done and why as far as alignment is concerned, and to get
> doc/README.arm-unaligned-accesses is clear and consistent with it.

I agree that we need to get on the same page and understand what is and
isn't (and is and cannot be done for us) going on, so we can get back to
bigger problems that need solving.

I wonder if we shouldn't largely crib from
https://www.kernel.org/doc/Documentation/unaligned-memory-access.txt and
modify the examples to be relevant to our sources, and perhaps drop the
networking section (as that doesn't directly apply).

And as an aside, building with -Wcast-align shows some places we _may_
need to investigate.

[snip]
> +ii) In order to make sure the following is self-sufficient, it goes
> +through the basics of alignment and assumes only good, not expert,
> +knowledge of the C language.
> +
> +1. C99 alignment requirements
> +
> +The C99 standard [1] (henceforth: 'C99') defines alignment requirements
> +as a "requirement that objects of a particular type be located on
> +storage boundaries with addresses that are particular multiples of a
> +byte address".
> +
> +In C99, unaligned accesses (those which which do not respect alignment
> +requirements of the object type being accessed) are deemed 'undefined'.
> +This means programs which contain unaligned accesses might build and
> +execute as if there were no alignment constraints, or build and execute
> +but not as expected, or build and crash at execution, or not build
> +at all--or even crash the compiler.

OK.

> +2. Implementation alignment requirements
> +
> +While C99 does define alignment requirements, it does not lay out any
> +actual alignment requirements, because these depend greatly on the
> +hardware involved; they are thus defined by C99 implementations.
> +
> +For ARM, the C alignment requirements are laid out in the ARM EABI [2].
> +For instance, is is the ARM EABI which sets the alignment constraint of
> +type 'long' to four-byte boundaries.
> +
> +Alignment requirements for a given architecture may differ from
> +hardware capabilities, i.e. they might be stricter than what the
> +hardware can actually do. One example is (32-bit) x86, which can do
> +unaligned accesses at the hardware level at some performance cost, but
> +has stricter requirement analogous to the ARM EABI ones. ARM is a mixed
> +bag: some older ARM hardware cannot perform unaligned access at all;
> +some can but at a cost; some can at virtually no cost.
> +
> +Further, even when a given architecture is capable of emitting such
> +unaligned accesses at the core or CPU level, at a higher (system) level
> +they might be forbidden because the target address falls within a
> +region in which only aligned accesses are possible.

The compiler must make reasonable decisions based on what it knows about
a particular architecture and ABI based on what it can know. The
developer must make correct decisions based on things the compiler
cannot know such as memory region constraints.

> +3. Native vs emulated unaligned accesses.
> +
> +There is a different between the alignment of accesses in a C program
> +source code and the alignment of accesses at the core or CPU level. In
> +the following, translated accesses (core or CPU accesses) will be
> +qualified as /native/ accesses to distinguish them from (untranslated) C
> +program accesses.
> +
> +A C99 implementation might translate an unaligned access into a native
> +unaligned access, or it might emulate it by a combination of native
> +aligned accesses.

This isn't relevant, given the contraint on the compiler to do sane
things for a given architecture.

> +4. Alignment requirements in U-Boot
> +
> +As U-Boot runs on a variety of hardware using a variety of C99
> +implementations, alignment requirements for the U-Boot code are the
> +strictest possible so that all implementations building U-Boot will have
> +their requirements satisfied. For instance, aligning longs to four-byte
> +boundaries is compatible with all implementations and is thus the
> +requirement for U-Boot.
> +
> +[U-Boot alignment requirements are actually stricter than just C99: in
> +U-Boot, at least some accesses of a given width must not be broken into
> +smaller accesses or lumped into a wider access, because they are done
> +to or from a device for which each physical access may have side
> +effects. C99 does not explicitly state such a constraint except in
> +general terms, that a C99 implementation .]
> +
> +The strict alignment requirements imply that all type declarations in
> +U-Boot should respect alignment requirements; especially, that no
> +structure should contain unaligned members.

This is where I moderately disagree. I agree we don't want to make
things unaligned for no reason in data structures.

> +5. Purposeful unaligned accesses in U-Boot
> +
> +However, there might be cases where some data representation may need
> +to include objects which are not aligned to their requirements. This
> +happens in structs representing protocols such as IP or USB, for
> +instance, or de facto standards such as FAT. Taking FAT as an example,

Right. We should combine 4 and 5 to an extent. We don't normally want,
but have cases we end up with it.

> +here is the start of the FAT header, expressed as a succession of C
> +types from position 0 in the first disk sector:
> +
> + 0 3 jump instruction
> + 3 8 Name of the tool which formatted the FAT
> + 11 2 Number of bytes per sector
> +
> +Since the third field is a short, as per ARM EABI it should be
> +even-aligned. However, it is not: its offset is 11, an odd value.
> +Therefore, a struct representing the FAT header (examples below are
> +adapted from actual code in fs/fdos/dos.h) would either have to split
> +the field in two smaller bytes...
> +
> + typedef struct bootsector
> + {
> + unsigned char jump [3]; /* 0 Jump to boot code */
> + char banner[BANNER_LG]; /* 3 OEM name & version */
> + unsigned char secsiz_lo; /* 11 Bytes per sector LO */
> + unsigned char secsiz_hi; /* 12 Bytes per sector HI */
> + [...]
> + } Directory_t;
> +
> +... but this is awkward and makes accesses to the field more
> +complicated to express and read. The alternative is to use a short
> +field, made unaligned by way of a 'packed' attribute on the struct:
> +
> + typedef struct bootsector
> + {
> + unsigned char jump [3]; /* 0 Jump to boot code */
> + char banner[BANNER_LG]; /* 3 OEM name & version */
> + unsigned short secsiz; /* 11 Bytes per sector */
> + [...]
> + } __attribute__ ((packed)) Directory_t;
> +
> +The latter is what is done in the U-Boot code, and the access is then
> +explicitly turned from one unaligned 16-bit to two aligned 8-bit ones
> +by using the macro __le16_to_cpu (which is slightly below U-Boot
> +requirements, as we should be using put_unaligned_le16 instead).

FWIW, this doesn't match the U-Boot sources where we just let the
compiler do padding and now that I type that I wonder if that's not part
of the blame for some of our fatwrite problems?

But here's the non-problem, demonstrated with the 'gpt write' code on an
armv5te target. Having described a struct to the compiler, and given
the compiler knows architecture aligment requirements it will either go:
(a) The struct is not packed, invisble padding time
(b) The struct is packed, this is unaligned, do it correctly.

> +6. Unintended unaligned accesses in U-Boot

We indeed don't want to allow actually bad things to be written and
work. On processors such as ARM where we may, or may not, do a silent
fixup, we choose not to do a silent fixup and instead fail. For
example, the following blows right up as we want it to today, and
continues to blow up with the patch I sent where we send
-mno-unaligned-access. It _only_ works if we cleared the SCTRL.A bit
which _we_do_not_want_.

diff --git a/common/cmd_misc.c b/common/cmd_misc.c
index 93f9eab..fa4a146 100644
--- a/common/cmd_misc.c
+++ b/common/cmd_misc.c
@@ -38,6 +38,18 @@ U_BOOT_CMD(
" - delay execution for N seconds (N is _decimal_ !!!)"
);

+static int do_unaligned(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
+{
+ unsigned char buf[8] = { 0 };
+ return *(int*)((unsigned long)buf | 1);
+}
+
+U_BOOT_CMD(
+ unaligned, 1, 1, do_unaligned,
+ "Make some unaligned accesses happen",
+ "Make some unaligned accesses happen"
+);
+
#ifdef CONFIG_CMD_TIMER
static int do_timer(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
{

And that's where we need to stop, more or less. The above is the type
of unaligned access problem we care about because it fails, everywhere
_unless_ we fix it up, and we don't want to. All of the problems we're
encountering now are either:
- Self inflicted (due to the mismatch in what the compiler requires and
what we've not done)
- Potentially abusing the packed attribute.

In the case of the GPT code, I'm still not convinced there's a problem
that isn't due to how we're building code that validly describes the
standard and how we tell the compiler to build it. It should be
__packed to match the on-disk format. Being __packed means the compiler
does the right thing about access or it's a compiler bug.

--
Tom
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