Re: [PATCH] KVM: MIPS/Emulate: Fix preemption warning

On Tue, Mar 14, 2017 at 12:05:15AM +0000, James Hogan wrote:

Host kernels with preemption enabled will emit the following preemption
warning when the guest FPU is used:

BUG: using smp_processor_id() in preemptible [00000000] code: qemu-system-mip/1495
caller is kvm_mips_emulate_CP0+0xa48/0xea0 [kvm]
...
Call Trace:
[<ffffffff81122d50>] show_stack+0x88/0xa8
[<ffffffff814307ac>] dump_stack+0x9c/0xd0
[<ffffffff81459f24>] check_preemption_disabled+0x124/0x130
[<ffffffffc000e908>] kvm_mips_emulate_CP0+0xa48/0xea0 [kvm]
[<ffffffffc000f9f0>] kvm_mips_emulate_inst+0x190/0x268 [kvm]
[<ffffffffc0016220>] kvm_trap_emul_handle_cop_unusable+0x48/0x160 [kvm]
[<ffffffffc000c120>] kvm_mips_handle_exit+0x520/0x7e0 [kvm]

This is due to the use of current_cpu_data.fpu_id from a preemptible
context to read the MIPS_FPIR_F64 bit. We don't currently support
asymmetric FPU capabilities with KVM, so just use boot_cpu_data instead
to silence the warning.

Fixes: 6cdc65e31d4f ("MIPS: KVM: Emulate FPU bits in COP0 interface")
Signed-off-by: James Hogan <redacted>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Radim Krčmář" <redacted>
Cc: Ralf Baechle <redacted>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org
Cc: <redacted> # 4.1.x-

Acked-by: Ralf Baechle <redacted>

James, we've fixed too many of these warnings over the past years, we
should look for a solution to kill this whole class of bugs.

  Ralf