gdbstub is broken on qemu-system-riscv64 with KVM enabled

Host environment

• Operating system: Ubuntu/Linux
• OS/kernel version: Linux ubuntu 6.6.21-ganboing #14 SMP PREEMPT_DYNAMIC Tue Jun 3 16:12:55 PDT 2025 riscv64 riscv64 riscv64 GNU/Linux
• Architecture: RISCV
• QEMU flavor: qemu-system-riscv64
• QEMU version: 9.2.4
• QEMU command line:

qemu-system-riscv64 --enable-kvm -cpu rv64,pmp=off -machine virt -smp 4 -nographic -m size=3072 -bios none

Emulated/Virtualized environment

• Operating system: seL4
• OS/kernel version: 13.0.0
• Architecture: RISCV

Description of problem

On RISCV Linux with KVM enabled, gdbstub is broken. The get_physical_address isn't able to page-walk correctly and resolve the physical page. This is due to that the vcpu is being treated as starting in M mode even if KVM enabled. However, with KVM, the vcpu is actually started in S mode. The mmu_idx will give 3 (M), instead of 1 (S), resulting in Guest PA == VA (wrong)! I did a hack to set the correct vcpu mode in the following patch, and the problem went away:

diff --git a/target/riscv/kvm/kvm-cpu.c b/target/riscv/kvm/kvm-cpu.c
index 6d1284c42f..880200659f 100644
--- a/target/riscv/kvm/kvm-cpu.c
+++ b/target/riscv/kvm/kvm-cpu.c
@@ -1653,6 +1653,7 @@ void kvm_riscv_reset_vcpu(RISCVCPU *cpu)
     env->scause = 0;
     env->stval = 0;
     env->mip = 0;
+    env->priv = PRV_S;
 }
 
 void kvm_riscv_set_irq(RISCVCPU *cpu, int irq, int level)

Additional information

I suspect that we never tried gdbstub with KVM enabled, because apart from this get_physical_address issue, I also need to turn off PMP (with -cpu rv64,pmp=off), otherwise, get_physical_address_pmp will also bail out due to no PMP configured (no permission). We probably need a comprehensive test and ensure gdbstub works with KVM. Thanks.