target/riscv/kvm: fix env->priv setting in reset_regs_csr()

This patch was originally made by Gitlab user Bo Gan (@ganboing) 4
months ago in the context of issue [1]. I asked the author to send a
patch to the mailing list ~3 months ago and got no reply. I'm sending
the patch myself because we already missed 10.1 without this fix.

I'll also just post verbatim Bo Gan comment in the commit msg:

"In 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)!"

Set env->priv to PRV_S in kvm_riscv_reset_regs_csr() since the VCPU is
always started in S-mode for KVM.

[1] https://gitlab.com/qemu-project/qemu/-/issues/2991

Cc: qemu-stable@nongnu.org
Closes: https://gitlab.com/qemu-project/qemu/-/issues/2991
Originally-by: Bo Gan (@ganboing in Gitlab)
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20251022111105.483992-1-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/kvm/kvm-cpu.c

@@ -705,6 +705,7 @@ static void kvm_riscv_reset_regs_csr(CPURISCVState *env)
     env->satp = 0;
     env->scounteren = 0;
     env->senvcfg = 0;
+    env->priv = PRV_S;
 }
 
 static int kvm_riscv_get_regs_fp(CPUState *cs)