seL4 / seL4

I am looking at USER_TOP in the generated configs in Microkit and I notice that all of them have the same value for USER_TOP which is 0xa0000000. This surprised me since this is quite low for all the platforms currently supported by Microkit.

I notice that if I build sel4test, e.g:
../init-build -DPLATFORM=odroidc4 && ninja

and look at kernel/gen_config/kernel/gen_config.h I see:

#define CONFIG_USER_TOP  0xa0000000

To my understanding, USER_TOP is not something configurable by the user of seL4 and so I don't think it's something going wrong with how I am building sel4test/Microkit?

Okay if I look at kernel_all.i I see in decodeARMPageTableInvocation the value of USER_TOP after pre-processing is 0x00007fffffffffff, which makes more sense.

This means there must be mismatch between USER_TOP used in the kernel code and what is exposed to user-space.

Wonderful, it looks like that

seL4/src/plat/qemu-arm-virt/config.cmake

Line 12 in 3740c29

set(qemu_user_top 0xa0000000)

leaks out into every platform because of

seL4/src/plat/qemu-arm-virt/config.cmake

Line 274 in 3740c29

DEFAULT ${qemu_user_top}

.

Looks like UNDEF_DISABLED needs to be added to that config declaration.

Looks like UNDEF_DISABLED needs to be added to that config declaration.

Won't that remove the config from being exported?

(I would like to use USER_TOP in code/tools)

The option you linked to is only declared for the qemu platform, so on other platforms it shouldn't be being declared as USER_TOP that the kernel uses isn't known by the build system apart from the qemu platform on aarch32: https://github.com/seL4/seL4/blob/master/libsel4/sel4_plat_include/qemu-arm-virt/sel4/plat/api/constants.h#L28

seL4_UserTop is supposed to be declared in libsel4.h for all platforms. So it's in a similar situation to the object size constants that are also defined in header files only.

Okay thanks. Will add the UNDEF_DISABLED and making seL4_UserTop exported in the other config files is a separate issue.

USER_TOP lower than expected on AArch64 platforms?