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4e5b0e8003
When building a 32-bit vDSO for a 64-bit kernel, games are played with
CONFIG_X86_64. {this,raw}_cpu_read_8() macros are conditionally defined
on CONFIG_X86_64 and when CONFIG_X86_64 is undefined in fake_32bit_build.h
various build failures in generic percpu header files can happen. To make
things worse, the build of 32-bit vDSO for a 64-bit kernel grew dependency
on arch_raw_cpu_ptr() macro and the build fails if arch_raw_cpu_ptr()
macro is not defined.
To mitigate these issues, x86 carefully defines arch_raw_cpu_ptr() to
avoid any dependency on raw_cpu_read_8() and thus CONFIG_X86_64. W/o
segment register support, the definition uses size-agnostic MOV asm
mnemonic and hopes that _ptr argument won't ever be 64-bit size on
32-bit targets (although newer GCCs warn for this situation with
"unsupported size for integer register"), and w/ segment register
support the definition uses size-agnostic __raw_cpu_read() macro.
Fortunately, raw_cpu_read() is not used in 32-bit vDSO for a 64-bit kernel.
However, we can't simply omit the definition of arch_raw_cpu_read(),
since the build will fail when building vdso/vdso32/vclock_gettime.o.
The patch defines arch_raw_cpu_ptr to BUILD_BUG() when BUILD_VDSO32_64
macro is defined. This way, we are sure that arch_raw_cpu_ptr() won't
actually be used in 32-bit VDSO for a 64-bit kernel, but it is still
defined to prevent build failure.
Finally, we can unify arch_raw_cpu_ptr() between builds w/ and w/o
x86 segment register support, substituting two tricky macro definitions
with a straightforward implementation.
There is no size difference and no difference in number of this_cpu_off
accesses between patched and unpatched kernel when the kernel is built
either w/ and w/o segment register support.
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20240322102730.209141-1-ubizjak@gmail.com
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the ReStructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.