VLLDM/VLSTM trigger UsageFault in the Secure Mode

[YVT]

The VLLDM/VLSTM instructions trigger UsageFault when they are supposed to behave as NOP.

Version:
$ qemu-system-arm --version QEMU emulator version 2.11.93

VLLDM and VLSTM are instructions newly added to ARMv8-M Mainline Profile. Although they are FP instructions and the FP support of the M profile is not implemented by QEMU, the Armv8-M Architecture Reference Manual specifies that they should behave as NOP even in this case:

C2.4.268 VLLDM:

> If the Floating-point Extension is not implemented, this instruction is available in Secure state, but behaves as a NOP.

C2.4.269 VLSTM:

> If the Floating-point Extension is not implemented, this instruction is available in Secure state, but behaves as a NOP.

VLLDM and VLSTM are generated automatically by the compiler to save and restore the floating point registers (in a lazy manner) during a Non-Secure function call. An example is shown below:

10000064 <__gnu_cmse_nonsecure_call>:
10000064: e92d 4fe0 stmdb sp!, {r5, r6, r7, r8, r9, sl, fp, lr}
10000068: 4627 mov r7, r4
1000006a: 46a0 mov r8, r4
1000006c: 46a1 mov r9, r4
1000006e: 46a2 mov sl, r4
10000070: 46a3 mov fp, r4
10000072: 46a4 mov ip, r4
10000074: b0a2 sub sp, #136 ; 0x88
10000076: ec2d 0a00 vlstm sp
1000007a: f384 8800 msr CPSR_f, r4
1000007e: 4625 mov r5, r4
10000080: 4626 mov r6, r4
10000082: 47a4 blxns r4
10000084: ec3d 0a00 vlldm sp
10000088: b022 add sp, #136 ; 0x88
1000008a: e8bd 8fe0 ldmia.w sp!, {r5, r6, r7, r8, r9, sl, fp, pc}

[Peter Maydell (pmaydell)]

Yes, you're right -- I hadn't noticed this wrinkle of the architecture. I'll put this on my todo list -- it should be straightforward.

Do you have a convenient test binary that I could use as a test case?

[YVT (yvt)]

I attached a ZIP file containing a set of binary images that reproduces the problem.

Secure.elf and NonSecure.elf contain codes that run in the Secure/Non-Secure mode, respectively. They must be loaded simultaneously by using the generic loader:

    $ qemu-system-arm -device loader,file=Secure.elf -kernel NonSecure.elf -machine mps2-an505 -nographic -s -cpu cortex-m33

The problematic instructions are located in 0x10000064 <__gnu_cmse_nonsecure_call> of Secure.elf. The program runs successfully and outputs some message via UART0 if they are replaced with NOPs, as shown below:

    $ qemu-system-arm -device loader,file=Secure-patched.elf -kernel NonSecure.elf -machine mps2-an505 -nographic -s -cpu cortex-m33
    I'm running in the Non-Secure mode.

[Peter Maydell (pmaydell)]

Submitted this patch for review which should fix this bug:
https://patchwork.ozlabs.org/patch/907959/

[Peter Maydell (pmaydell)]

Fix now in master as commit b1e5336a9899016c53d59 (and cc stable), so should be in 3.0 and 2.12.1.