[Qemu-devel] [PATCH v1] ppc: Force CPU threads count to be a power of 2.

[Qemu-devel] [PATCH v1] ppc: Force CPU threads count to be a power of 2.

[Bharata B Rao]

PowerPC kernel expects the number of SMT threads in a core to be a power
of 2. Since QEMU doesn't enforce this, it leads to an early guest kernel
crash if invalid threads count is specified.

Prevent this crash and make it a graceful exit from QEMU itself by
validating the user supplied threads count.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Acked-by: Stewart Smith <stewart@linux.vnet.ibm.com>
---
Changes in v1: Make error message more descriptive as per Stewart's suggestion.
v0: https://lists.gnu.org/archive/html/qemu-devel/2014-03/msg00355.html

 target-ppc/translate_init.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/target-ppc/translate_init.c b/target-ppc/translate_init.c
index 445c360..9ed22bb 100644
--- a/target-ppc/translate_init.c
+++ b/target-ppc/translate_init.c
@@ -18,6 +18,7 @@
  * License along with this library; if not, see <http://www.gnu.org/licenses/>.
  */
 
+#include <math.h>
 #include "disas/bfd.h"
 #include "exec/gdbstub.h"
 #include <sysemu/kvm.h>
@@ -7979,6 +7980,7 @@ static void ppc_cpu_realizefn(DeviceState *dev, Error **errp)
     Error *local_err = NULL;
 #if !defined(CONFIG_USER_ONLY)
     int max_smt = kvm_enabled() ? kvmppc_smt_threads() : 1;
+    int threads_shift;
 #endif
 
 #if !defined(CONFIG_USER_ONLY)
@@ -7987,6 +7989,13 @@ static void ppc_cpu_realizefn(DeviceState *dev, Error **errp)
                    max_smt, kvm_enabled() ? "KVM" : "TCG");
         return;
     }
+    threads_shift = log2(smp_threads);
+    if (smp_threads != (1 << threads_shift)) {
+        error_setg(errp, "Cannot support %d threads on PPC with %s, "
+                   "threads count must be a power of 2.",
+                   smp_threads, kvm_enabled() ? "KVM" : "TCG");
+        return;
+    }
 #endif
 
     if (kvm_enabled()) {
-- 
1.7.11.7

Re: [Qemu-devel] [PATCH v1] ppc: Force CPU threads count to be a power of 2.

[Eric Blake]

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On 03/04/2014 08:36 PM, Bharata B Rao wrote:
> PowerPC kernel expects the number of SMT threads in a core to be a power
> of 2. Since QEMU doesn't enforce this, it leads to an early guest kernel
> crash if invalid threads count is specified.
> 
> Prevent this crash and make it a graceful exit from QEMU itself by
> validating the user supplied threads count.
> 

>  
> +#include <math.h>

>      }
> +    threads_shift = log2(smp_threads);

Overkill.  qemu-common.h gives you is_power_of_2() that uses just
integer math rather than dragging in floating-point overhead of libm.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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Re: [Qemu-devel] [PATCH v1] ppc: Force CPU threads count to be a power of 2.

[Bharata B Rao]

On Tue, Mar 04, 2014 at 09:13:38PM -0700, Eric Blake wrote:
> On 03/04/2014 08:36 PM, Bharata B Rao wrote:
> > PowerPC kernel expects the number of SMT threads in a core to be a power
> > of 2. Since QEMU doesn't enforce this, it leads to an early guest kernel
> > crash if invalid threads count is specified.
> > 
> > Prevent this crash and make it a graceful exit from QEMU itself by
> > validating the user supplied threads count.
> > 
> 
> >  
> > +#include <math.h>
> 
> >      }
> > +    threads_shift = log2(smp_threads);
> 
> Overkill.  qemu-common.h gives you is_power_of_2() that uses just
> integer math rather than dragging in floating-point overhead of libm.

Nice. Using is_power_of_2() in v2.

Regards,
Bharata.