Re: [PATCH] x86: kvm: introduce CONFIG_KVM_MAX_VCPUS

[Andrew Jones]

On Mon, Sep 16, 2013 at 05:16:20PM +0300, Gleb Natapov wrote:
> On Mon, Sep 16, 2013 at 02:03:33PM +0200, Andrew Jones wrote:
> > 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 was talking about configuring more then 255 vcus in kernel config. You
> should have "range 1 255" there.

Ah, I assumed one could configure this higher if they wanted to test
something higher. Other than requiring another word in a bitmap, I didn't
see what would change going beyond that boundary.

> 
> > 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.
> > 
> The idea behind soft/hard limit was to allow people easily check
> scalability without need to recompile the kernel. If custom build will
> be needed most people will not bother.

Keeping the default 255 wouldn't have changed anything we have today,
other than making it easier to adjust for scalability testing beyond 255
(assuming they could go beyond). However, as I just wrote in the other
thread, you've convinced me to keep a constant soft limit definition, but
since the soft limit is a custom concept, then I believe it belongs in the
config.

drew
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