On Thu, 7 Nov 2019, Ingo Molnar wrote:
> * Thomas Gleixner <tglx@linutronix.de> wrote:
> >  	/*
> > -	 * The extra 1 is there because the CPU will access an
> > -	 * additional byte beyond the end of the IO permission
> > -	 * bitmap. The extra byte must be all 1 bits, and must
> > -	 * be within the limit.
> > +	 * The extra 1 is there because the CPU will access an additional
> > +	 * byte beyond the end of the I/O permission bitmap. The extra byte
> > +	 * must have all bits set and must be within the TSS limit.
> >  	 */
> >  	unsigned long		io_bitmap[IO_BITMAP_LONGS + 1];
> >  } __aligned(PAGE_SIZE);
> 
> Note that on 32-bit kernels this blows up our CPU area calculations:
> 
> ./include/linux/compiler.h:350:38: error: call to ‘__compiletime_assert_181’ declared with attribute error: BUILD_BUG_ON failed: CPU_ENTRY_AREA_PAGES * PAGE_SIZE < CPU_ENTRY_AREA_MAP_SIZE
> ./include/linux/compiler.h:331:4: note: in definition of macro ‘__compiletime_assert’
> ./include/linux/compiler.h:350:2: note: in expansion of macro ‘_compiletime_assert’
> ./include/linux/build_bug.h:39:37: note: in expansion of macro ‘compiletime_assert’
> ./include/linux/build_bug.h:50:2: note: in expansion of macro ‘BUILD_BUG_ON_MSG’
> arch/x86/mm/cpu_entry_area.c:181:2: note: in expansion of macro ‘BUILD_BUG_ON’
> make[2]: *** [scripts/Makefile.build:265: arch/x86/mm/cpu_entry_area.o] Error 1

Duh. /me goes investigate.