On Mon, Feb 24, 2025 at 02:39:59PM +0530, Aneesh Kumar K.V (Arm) wrote:
> The return value for kernel VM exit handlers is confusing and has led to
> errors in different kernel exit handlers. A return value of 0 indicates
> a return to the VMM, whereas a return value of 1 indicates resuming
> execution in the guest. Some handlers mistakenly return 0 to force a
> return to the guest.
>
> This worked in kvmtool because the exit_reason defaulted to
> 0 (KVM_EXIT_UNKNOWN), and kvmtool did not error out on an unknown exit
> reason. However, forcing a VMM exit with error on KVM_EXIT_UNKNOWN
> exit_reson would help catch these bugs early.
>
> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@xxxxxxxxxx>
> ---
> kvm-cpu.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kvm-cpu.c b/kvm-cpu.c
> index f66dcd07220c..7c62bfc56679 100644
> --- a/kvm-cpu.c
> +++ b/kvm-cpu.c
> @@ -170,7 +170,7 @@ int kvm_cpu__start(struct kvm_cpu *cpu)
>
> switch (cpu->kvm_run->exit_reason) {
> case KVM_EXIT_UNKNOWN:
> - break;
> + goto panic_kvm;
> case KVM_EXIT_DEBUG:
> kvm_cpu__show_registers(cpu);
> kvm_cpu__show_code(cpu);
> --
> 2.43.0
This breaks SMP boot on my x86 machine:
# ./lkvm run
...
[ 0.628472] smp: Bringing up secondary CPUs ...
[ 0.630401] smpboot: x86: Booting SMP configuration:
Error: KVM exit reason: 0 ("KVM_EXIT_UNKNOWN")
Error: KVM exit code: 0
Registers:
----------
rip: 0000000000000000 rsp: 0000000000000000 flags: 0000000000000002
rax: 0000000000000000 rbx: 0000000000000000 rcx: 0000000000000000
rdx: 0000000000050654 rsi: 0000000000000000 rdi: 0000000000000000
rbp: 0000000000000000 r8: 0000000000000000 r9: 0000000000000000
r10: 0000000000000000 r11: 0000000000000000 r12: 0000000000000000
r13: 0000000000000000 r14: 0000000000000000 r15: 0000000000000000
cr0: 0000000060000010 cr2: 0000000000000000 cr3: 0000000000000000
cr4: 0000000000000000 cr8: 0000000000000000
Segment registers:
------------------
register selector base limit type p dpl db s l g avl
cs 9900 0000000000099000 0000ffff 0b 1 0 0 1 0 0 0
ss 0000 0000000000000000 0000ffff 03 1 0 0 1 0 0 0
ds 0000 0000000000000000 0000ffff 03 1 0 0 1 0 0 0
es 0000 0000000000000000 0000ffff 03 1 0 0 1 0 0 0
fs 0000 0000000000000000 0000ffff 03 1 0 0 1 0 0 0
gs 0000 0000000000000000 0000ffff 03 1 0 0 1 0 0 0
tr 0000 0000000000000000 0000ffff 0b 1 0 0 0 0 0 0
ldt 0000 0000000000000000 0000ffff 02 1 0 0 0 0 0 0
gdt 0000000000000000 0000ffff
idt 0000000000000000 0000ffff
APIC:
-----
efer: 0000000000000000 apic base: 00000000fee00800 nmi: enabled
Interrupt bitmap:
-----------------
0000000000000000 0000000000000000 0000000000000000 0000000000000000
Code:
-----
Warning: symbol_lookup() failed to find symbol with error: -2
rip: [<0000000000000000>] <unknown>
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 <fa> 0f 09 ea 08 10 00 98 8c c8 8e d8 8e c0 8e d0 66 f0 0f ba 2e
Stack:
------
rsp: [<0000000000000000>]
0x00000000: 30 00 00 f0 30 00 00 f0
0x00000008: 30 00 00 f0 30 00 00 f0
0x00000010: 30 00 00 f0 30 00 00 f0
0x00000018: 30 00 00 f0 30 00 00 f0
Page Tables:
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Not in protected mode
