Re: [Qemu-devel] [PATCH v2] accel: forbid early use of kvm_enabled() and friends

[Paolo Bonzini]

On 29/06/2018 12:29, Greg Kurz wrote:
> It is unsafe to rely on *_enabled() helpers before the accelerator has
> been initialized, ie, accel_init_machine() has succeeded, because they
> always return false. But it is still possible to end up calling them
> indirectly by inadvertance, and cause QEMU to misbehave.
> 
> This patch causes QEMU to abort if we try to check for an accelerator
> before it has been set up. This will help to catch bugs earlier.
> 
> Signed-off-by: Greg Kurz <address@hidden>
> Reviewed-by: David Gibson <address@hidden>
> ---
> 
> This patch was motivated by an regression we're currently fixing in
> spapr because of an early use of kvm_enabled(). David suggested to
> post this patch separately:
> 
> https://lists.nongnu.org/archive/html/qemu-ppc/2018-06/msg01136.html
> 
> v2: - dropped change in qom/cpu.c (useless header inclusion)
>     - only #include "sysemu/kvm.h" if we actually need it
>     - added David's R-b from v1 because changes in v2 are minor

This adds a function call on possibly hot paths.  Can you make it inline?

Also asserting current_machine != NULL is not necessary, since you're
immediately dereferencing it.

Thanks,

Paolo

> ---
>  accel/accel.c          |    7 +++++++
>  include/qemu-common.h  |    3 ++-
>  include/sysemu/accel.h |    1 +
>  include/sysemu/kvm.h   |    3 ++-
>  stubs/Makefile.objs    |    1 +
>  stubs/accel.c          |   14 ++++++++++++++
>  target/i386/hax-all.c  |    2 +-
>  target/i386/whpx-all.c |    2 +-
>  8 files changed, 29 insertions(+), 4 deletions(-)
>  create mode 100644 stubs/accel.c
> 
> diff --git a/accel/accel.c b/accel/accel.c
> index 966b2d8f536c..27900aac9cc5 100644
> --- a/accel/accel.c
> +++ b/accel/accel.c
> @@ -51,6 +51,13 @@ static AccelClass *accel_find(const char *opt_name)
>      return ac;
>  }
>  
> +bool assert_accelerator_initialized(bool allowed)
> +{
> +    assert(current_machine != NULL);
> +    assert(current_machine->accelerator != NULL);
> +    return allowed;
> +}
> +
>  static int accel_init_machine(AccelClass *acc, MachineState *ms)
>  {
>      ObjectClass *oc = OBJECT_CLASS(acc);
> diff --git a/include/qemu-common.h b/include/qemu-common.h
> index 85f4749aefb7..01d5e4d97dbf 100644
> --- a/include/qemu-common.h
> +++ b/include/qemu-common.h
> @@ -82,7 +82,8 @@ int qemu_openpty_raw(int *aslave, char *pty_name);
>  extern bool tcg_allowed;
>  void tcg_exec_init(unsigned long tb_size);
>  #ifdef CONFIG_TCG
> -#define tcg_enabled() (tcg_allowed)
> +#include "sysemu/accel.h"
> +#define tcg_enabled() (assert_accelerator_initialized(tcg_allowed))
>  #else
>  #define tcg_enabled() 0
>  #endif
> diff --git a/include/sysemu/accel.h b/include/sysemu/accel.h
> index 637358f43014..76965cb69cc9 100644
> --- a/include/sysemu/accel.h
> +++ b/include/sysemu/accel.h
> @@ -71,5 +71,6 @@ void configure_accelerator(MachineState *ms);
>  void accel_register_compat_props(AccelState *accel);
>  /* Called just before os_setup_post (ie just before drop OS privs) */
>  void accel_setup_post(MachineState *ms);
> +bool assert_accelerator_initialized(bool allowed);
>  
>  #endif
> diff --git a/include/sysemu/kvm.h b/include/sysemu/kvm.h
> index 0b64b8e06786..5a2e59e99128 100644
> --- a/include/sysemu/kvm.h
> +++ b/include/sysemu/kvm.h
> @@ -46,7 +46,8 @@ extern bool kvm_direct_msi_allowed;
>  extern bool kvm_ioeventfd_any_length_allowed;
>  extern bool kvm_msi_use_devid;
>  
> -#define kvm_enabled()           (kvm_allowed)
> +#include "sysemu/accel.h"
> +#define kvm_enabled()           (assert_accelerator_initialized(kvm_allowed))
>  /**
>   * kvm_irqchip_in_kernel:
>   *
> diff --git a/stubs/Makefile.objs b/stubs/Makefile.objs
> index 53d3f32cb258..2d5142287525 100644
> --- a/stubs/Makefile.objs
> +++ b/stubs/Makefile.objs
> @@ -43,3 +43,4 @@ stub-obj-y += xen-common.o
>  stub-obj-y += xen-hvm.o
>  stub-obj-y += pci-host-piix.o
>  stub-obj-y += ram-block.o
> +stub-obj-y += accel.o
> diff --git a/stubs/accel.c b/stubs/accel.c
> new file mode 100644
> index 000000000000..4f480f2d3f29
> --- /dev/null
> +++ b/stubs/accel.c
> @@ -0,0 +1,14 @@
> +/*
> + * accel stubs
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2 or later.
> + * See the COPYING file in the top-level directory.
> + */
> +
> +#include "qemu/osdep.h"
> +#include "sysemu/accel.h"
> +
> +bool assert_accelerator_initialized(bool allowed)
> +{
> +    return allowed;
> +}
> diff --git a/target/i386/hax-all.c b/target/i386/hax-all.c
> index d2e512856bb8..7c78bd7d094d 100644
> --- a/target/i386/hax-all.c
> +++ b/target/i386/hax-all.c
> @@ -57,7 +57,7 @@ static int hax_arch_get_registers(CPUArchState *env);
>  
>  int hax_enabled(void)
>  {
> -    return hax_allowed;
> +    return assert_accelerator_initialized(hax_allowed);
>  }
>  
>  int valid_hax_tunnel_size(uint16_t size)
> diff --git a/target/i386/whpx-all.c b/target/i386/whpx-all.c
> index 6b42096698ee..e7f6bc5958e7 100644
> --- a/target/i386/whpx-all.c
> +++ b/target/i386/whpx-all.c
> @@ -1422,7 +1422,7 @@ static int whpx_accel_init(MachineState *ms)
>  
>  int whpx_enabled(void)
>  {
> -    return whpx_allowed;
> +    return assert_accelerator_initialized(whpx_allowed);
>  }
>  
>  static void whpx_accel_class_init(ObjectClass *oc, void *data)
>