Re: [PATCH] x86: kvm: introduce CONFIG_KVM_MAX_VCPUS

[Andrew Jones]

On Mon, Sep 16, 2013 at 11:47:10AM +0300, Gleb Natapov wrote:
> On Mon, Sep 16, 2013 at 10:28:20AM +0200, Andrew Jones wrote:
> > On Sun, Sep 15, 2013 at 12:08:38PM +0300, Gleb Natapov wrote:
> > > On Sat, Sep 14, 2013 at 02:18:49PM +0200, Andrew Jones wrote:
> > > > Take CONFIG_KVM_MAX_VCPUS from arm32, but set the default to 255.
> > > > 
> > > > Signed-off-by: Andrew Jones <drjones@xxxxxxxxxx>
> > > > ---
> > > >  arch/x86/include/asm/kvm_host.h |  5 +++--
> > > >  arch/x86/kvm/Kconfig            | 10 ++++++++++
> > > >  2 files changed, 13 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > > > index c76ff74a98f2e..e7e9b523a8f7e 100644
> > > > --- a/arch/x86/include/asm/kvm_host.h
> > > > +++ b/arch/x86/include/asm/kvm_host.h
> > > > @@ -31,8 +31,9 @@
> > > >  #include <asm/msr-index.h>
> > > >  #include <asm/asm.h>
> > > >  
> > > > -#define KVM_MAX_VCPUS 255
> > > > -#define KVM_SOFT_MAX_VCPUS 160
> > > > +#define KVM_MAX_VCPUS CONFIG_KVM_MAX_VCPUS
> > > > +#define KVM_SOFT_MAX_VCPUS min(160, KVM_MAX_VCPUS)
> > > > +
> > > >  #define KVM_USER_MEM_SLOTS 125
> > > >  /* memory slots that are not exposed to userspace */
> > > >  #define KVM_PRIVATE_MEM_SLOTS 3
> > > > diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> > > > index a47a3e54b964b..e9532c33527ee 100644
> > > > --- a/arch/x86/kvm/Kconfig
> > > > +++ b/arch/x86/kvm/Kconfig
> > > > @@ -52,6 +52,16 @@ config KVM
> > > >  
> > > >  	  If unsure, say N.
> > > >  
> > > > +config KVM_MAX_VCPUS
> > > > +	int "Number maximum supported virtual CPUs per VM"
> > > > +	depends on KVM
> > > > +	default 255
> > > > +	help
> > > > +	  Static number of max supported virtual CPUs per VM.
> > > > +
> > > > +	  Set to a lower number to save some resources. Set to a higher
> > > > +	  number to test scalability.
> > > > +
> > > Maximum this can save is around 2K per VM. This is pretty insignificant
> > > considering overall memory footprint even smallest VM has.
> > 
> > Should I reword this, dropping all 'save resources' verbiage, in order to
> > avoid sending a message that this option can affect resource consumption?
> > Or just leave it as it is, because even though it's insignificant, it's
> > still true and balances out the 'Set to a higher' part.
> > 
> I do not think config option is necessary. The overhead is so
> insignificant that there is no point in additional user visible knob,
> at least while only 255 vcpu are supported. Is there a reason for anyone
> to configure less them 255 vcpus here? OTOH what prevents someone from
> configuring more then 255 vcpus?

The reason to configure less is to be able to compile a hard limit,
without having to muck with the source. E.g. if you really don't want to
allow more than KVM_SOFT_MAX_VCPUS to be configured, then you can compile
with CONFIG_KVM_MAX_VCPUS == KVM_SOFT_MAX_VCPUS.

Nothing prevents someone from configuring more than the max in
userspace, but if they try to create/use more than the max
(kvm_vm_ioctl_create_vcpu), it'll fail (EINVAL).

I see this as a step towards getting rid of KVM_SOFT_MAX_VCPUS. I.e.
compile with whatever maximum limit you want (can support), and return
online/possible cpus for the recommended number. Only configure the kernel
with more than your typical maximum (was KVM_SOFT_MAX_VCPUS) for
development/testing purposes.

drew
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