[Qemu-devel] [PATCH] limit guest cpus for kvm enabled builds

[Qemu-devel] [PATCH] limit guest cpus for kvm enabled builds

[Bruce Rogers]

The x86 kvm kernel module limits guest cpu count to 16, but the userspace pc definition says 255 still.  When invoking qemu-kvm with -smp > 16, it prints some error messages, then segfaults.   One of the mistakes is that the error code from kvm_create_vcpu is ignored in ap_main_loop.
This patch will enforce the limit at initialization time and avoid these problems from the get go.

Bruce

Signed-off-by: Bruce Rogers <brogers@novell.com>

--- a/hw/pc.c
+++ b/hw/pc.c
@@ -1162,7 +1162,11 @@ QEMUMachine pc_machine = {
     .desc = "Standard PC",
     .init = pc_init_pci,
     .ram_require = VGA_RAM_SIZE + PC_MAX_BIOS_SIZE,
+#ifdef CONFIG_KVM
+    .max_cpus = 16,
+#else
     .max_cpus = 255,
+#endif
 };

Re: [Qemu-devel] [PATCH] limit guest cpus for kvm enabled builds

[Paul Brook]

On Thursday 09 April 2009, Bruce Rogers wrote:
> The x86 kvm kernel module limits guest cpu count to 16, but the userspace
> pc definition says 255 still.  When invoking qemu-kvm with -smp > 16, it
> prints some error messages, then segfaults.   One of the mistakes is that
> the error code from kvm_create_vcpu is ignored in ap_main_loop. 

You should fix that bug then.

> This patch will enforce the limit at initialization time and avoid these
> problems from the get go.

No. We still want to support a decent number of cpus when -no-kvm is 
specified.

Paul

Re: [Qemu-devel] [PATCH] limit guest cpus for kvm enabled builds

[Anthony Liguori]

Paul Brook wrote:
> On Thursday 09 April 2009, Bruce Rogers wrote:
>   
>> The x86 kvm kernel module limits guest cpu count to 16, but the userspace
>> pc definition says 255 still.  When invoking qemu-kvm with -smp > 16, it
>> prints some error messages, then segfaults.   One of the mistakes is that
>> the error code from kvm_create_vcpu is ignored in ap_main_loop. 
>>     
>
> You should fix that bug then.
>   

I don't see the patch on the mailing list yet, but I'm pretty sure your 
patch is against kvm-userspace.git as ap_main_loop doesn't exist in 
upstream QEMU.

And FWIW, we do properly handle an unsupported -smp value in upstream 
QEMU when using KVM.

Regards,

Anthony Liguori

Re: [Qemu-devel] [PATCH] limit guest cpus for kvm enabled builds

[Bruce Rogers]

 On 4/9/2009 at 10:44 AM, Anthony Liguori <anthony@codemonkey.ws> wrote:
> Paul Brook wrote:
>> On Thursday 09 April 2009, Bruce Rogers wrote:
>>   
>>> The x86 kvm kernel module limits guest cpu count to 16, but the userspace
>>> pc definition says 255 still.  When invoking qemu-kvm with -smp > 16, it
>>> prints some error messages, then segfaults.   One of the mistakes is that
>>> the error code from kvm_create_vcpu is ignored in ap_main_loop. 
>>>     
>>
>> You should fix that bug then.
>>   
> 
> I don't see the patch on the mailing list yet, but I'm pretty sure your 
> patch is against kvm-userspace.git as ap_main_loop doesn't exist in 
> upstream QEMU.
> 
> And FWIW, we do properly handle an unsupported -smp value in upstream 
> QEMU when using KVM.
> 
> Regards,
> 
> Anthony Liguori

OK thanks.
I've submitted a different patch on kvm mailing list.
- Bruce