On Thu, May 25, 2017 at 06:07:35AM -0700, Ard Biesheuvel wrote:
> On 25 May 2017 at 05:56, Lukas Wunner <lukas@xxxxxxxxx> wrote:
> > On Thu, May 25, 2017 at 05:47:59AM -0700, Ard Biesheuvel wrote:
> >> On 25 May 2017 at 05:44, Lukas Wunner <lukas@xxxxxxxxx> wrote:
> >> > On Thu, May 25, 2017 at 05:36:01AM -0700, Ard Biesheuvel wrote:
> >> >> On 25 May 2017 at 05:30, Lukas Wunner <lukas@xxxxxxxxx> wrote:
> >> >> > On Thu, May 11, 2017 at 03:06:42PM +0100, Ard Biesheuvel wrote:
> >> >> >> On 10 May 2017 at 09:41, Lukas Wunner <lukas@xxxxxxxxx> wrote:
> >> >> >> > On Wed, May 10, 2017 at 09:03:11AM +0100, Ard Biesheuvel wrote:
> >> >> >> >> On 6 May 2017 at 10:07, Lukas Wunner <lukas@xxxxxxxxx> wrote:
> >> >> >> >> > On Sat, May 06, 2017 at 08:46:07AM +0100, Ard Biesheuvel wrote:
> >> >> >> >> >> On 5 May 2017 at 19:38, Lukas Wunner <lukas@xxxxxxxxx> wrote:
> >> >> >> >> >> > The CPER parser assumes that the class code is big endian, but at least
> >> >> >> >> >> > on this edk2-derived Intel Purley platform it's little endian:
> >> >> >> >> > [snip]
> >> >> >> >> >> > --- a/include/linux/cper.h
> >> >> >> >> >> > +++ b/include/linux/cper.h
> >> >> >> >> >> > @@ -416,7 +416,7 @@ struct cper_sec_pcie {
> >> >> >> >> >> > struct {
> >> >> >> >> >> > __u16 vendor_id;
> >> >> >> >> >> > __u16 device_id;
> >> >> >> >> >> > - __u8 class_code[3];
> >> >> >> >> >> > + __u32 class_code:24;
> >> >> >> >> >>
> >> >> >> >> >> I'd like to avoid this change if we can. Couldn't we simply invert the
> >> >> >> >> >> order of p[] above?
> >> >> >> >> >
> >> >> >> >> > Hm, why would you like to avoid it?
> >> >> >> >>
> >> >> >> >> Because we shouldn't use bitfields in structs in code that should be
> >> >> >> >> portable across archs with different endiannesses.
> >> >> >> >
> >> >> >> > The CPER header is defined in the UEFI spec and UEFI mandates that the
> >> >> >> > arch is little endian (UEFI r2.6, sec. 2.3.5, 2.3.6).
> >> >> >> >
> >> >> >>
> >> >> >> No it does not mandate that at all. It mandates how the core should be
> >> >> >> configured when running in UEFI, but the OS can do anything it likes.
> >> >> >>
> >> >> >> We are still interested in adding limited UEFI support to big endian
> >> >> >> arm64 in the future (i.e., access to a limited set of firmware tables
> >> >> >> but no runtime services), and I am not going to merge anything that
> >> >> >> moves us away from that goal.
> >> >> >>
> >> >> >> > So your argument seems moot to me. Am I missing something? Do you
> >> >> >> > have another argument?
> >> >> >> >
> >> >> >> > Moreover, the vendor_id and device_id fields are little endian as well
> >> >> >> > (PCI r3.0, sec. 6.1), yet there are no provisions in our CPER parser in
> >> >> >> > drivers/firmware/efi/cper.c to convert them to the endianness of the host.
> >> >> >> >
> >> >> >>
> >> >> >> Indeed. I am aware we will need to add various endian-neutral
> >> >> >> accessors in the future.
> >> >> >>
> >> >> >> >> > The class_code element isn't
> >> >> >> >> > referenced anywhere else in the kernel and this isn't a uapi header,
> >> >> >> >> > so the change would only impact out-of-tree drivers. Not sure if
> >> >> >> >> > any exist which might be interested in CPER parsing.
> >> >> >> >> >
> >> >> >> >>
> >> >> >> >> The point is that the change in the struct definition is simply not
> >> >> >> >> necessary, given that inverting the order of p[] already achieves
> >> >> >> >> exactly what we want.
> >> >> >> >
> >> >> >> > It seems clumsy and unnecessary to me so I'd prefer the bitfield.
> >> >> >> > Please excuse my stubbornness.
> >> >> >> >
> >> >> >>
> >> >> >> Stubbornness alone is not going to convince me. What *could* convince
> >> >> >> me (although unlikely) is a quote from the C spec which explains why
> >> >> >> it is 100% legal to make assumptions about how bitfields are projected
> >> >> >> onto byte locations in memory.
> >> >> >
> >> >> > All structs in cper.h are declared "packed", so what you're asking for
> >> >> > isn't defined in the C spec but in the GCC documentation:
> >> >> >
> >> >> > "The packed attribute specifies that a variable or structure field
> >> >> > should have the smallest possible alignment -- one byte for a variable,
> >> >> > and one bit for a field, unless you specify a larger value with the
> >> >> > aligned attribute."
> >> >> >
> >> >> > So I maintain that the patch is fine, but you'll need to use le32_to_cpu(),
> >> >> > le16_to_cpu() etc both for the class_code changed by the patch as well as
> >> >> > all the other members of the struct not touched by the patch when adding
> >> >> > "endianness mixed mode" for aarch64.
> >> >>
> >> >> I'm not talking about the 'packed' attribute but about the fact that
> >> >> the C spec does not guarantee that bitfields are projected onto byte
> >> >> locations in memory in the way you expect.
> >> >
> >> > What relevance does that have as long as the header file uses a pragma
> >> > specific to gcc (or other compilers that are compatible to gcc with
> >> > respect to that pragma (such as clang)), and gcc guarantees the
> >> > correct layout regardless of endianness?
> >>
> >> The relevance is that we should not add GCC specific code because you
> >> think it looks prettier.
> >
> > The code already *is* gcc-specific.
> >
>
> The entire kernel is GCC specific. But that does not justify adding
> more GCC-isms throughout the code.
How is the patch adding a GCC-ism?
> >> And where does GCC guarantee the correct layout? Did you find an
> >> unambiguous GCC documentation reference that explains how bitfields
> >> are mapped onto byte locations?
> >
> > See the excerpt I quoted above.
> >
>
> 'packed' has nothing to do with it. This is about bitfields in structs.
'packed' has *everything* to do with it. :-)
The struct contains a *single* bitfield surrounded by non-bitfields.
If there were multiple consecutive bitfields, then yes, things wouldn't
be as clear.
The bitfield as well as all surrounding non-bitfields have a size which
is a multiple of full bytes. And this is where the 'packed' attribute
comes into play, it guarantees that there's no padding as long as all
members of the struct are byte-aligned.
>
> >
> >> Or does 'guarantee' mean 'I tested it and it works'?
> >
> > I tested it with x86_64 (le) and ppc32 (be) and it works.
> > I don't have an aarch64 machine available here.
> >
>
> Good.
Good to merge then?
Thanks,
Lukas
