RE: read_sysreg(TTBR_EL2) not work in linux, but read_sysreg(CurrentEL) works. (arm64)

[Chan Kim]

 

I used an old kernel (5.4.21) and I missed during the kernel build, the compiler complained that

“selected processor does not support system register name ‘ttbr1_el2’”.

So this reading of ttbr1_el2 seemed to have been ignored.

Switching to kernel 5.10.0rc, this error is not coming.

Thank you.

Chan Kim

 

From: Chan Kim <ckim@etri.re.kr>
Sent: Tuesday, February 22, 2022 3:15 PM
To: 'qemu-discuss@nongnu.org' <qemu-discuss@nongnu.org>
Subject: read_sysreg(TTBR_EL2) not work in linux, but read_sysreg(CurrentEL) works. (arm64)

 

When running u-boot starting linux on qemu and using gdb to debug the program, (on a modified arm64 virt machine),

I see this strange things.

Right after I enter linux start_kernel( ), I added these two lines,

datax = read_sysreg(CurrentEL);    -- line 1

datax = read_sysreg(TTBR1_EL2);    -- line 2

after line 1 and before line 2, the value of datax is seen as 8. (bit [3:2] is current EL, so it's EL2). 

after line 2, the value of datax is still 8, but the result of `info reg TTBR1_EL2` is 0x8065a000.

The disassembled assembly code shows no reading of TTBR1_EL2 register.

How can this happen? Should I set something in the kernel configuration?

Any help will be really appreciated.

Thank you!

Chan Kim