On Sun, Sep 15, 2013 at 12:03:22PM +0300, Gleb Natapov wrote: > On Sat, Sep 14, 2013 at 02:16:51PM +0200, Andrew Jones wrote: > > This patch removes KVM_SOFT_MAX_VCPUS and uses num_online_cpus() for > > KVM_CAP_NR_VCPUS instead, as ARM does. While the API doc simply says > > KVM_CAP_NR_VCPUS should return the recommended maximum number of vcpus, > > it has been returning KVM_SOFT_MAX_VCPUS, which was defined as the > > maximum tested number of vcpus. As that concept could be > > distro-specific, this patch uses the other recommended maximum, the > > number of physical cpus, as we never recommend configuring a guest that > > has more vcpus than the host has pcpus. Of course a guest can always > > still be configured with up to KVM_CAP_MAX_VCPUS though anyway. > > > > I've put RFC on this patch because I'm not sure if there are any gotchas > > lurking with this change. The change now means hosts no longer return > > the same number for KVM_CAP_NR_VCPUS, and that number is likely going to > > generally be quite a bit less than what KVM_SOFT_MAX_VCPUS was (160). I > > can't think of anything other than generating more warnings[1] from qemu > > with guests that configure more vcpus than pcpus though. > > > Another gotcha is that on a host with more then 160 cpus recommended > value will grow which is not a good idea without appropriate testing. Good point. Of course the objective could be to test a guest with vcpus > 160 on that host, and then the potential warning messages would need to be ignored. Probably the best place to set the cap on the number of vcpus used in a stable environment would be in KVM_MAX_VCPUS. That said, then at least until KVM_SOFT_MAX_VCPUS catches up to KVM_MAX_VCPUS, I guess we should keep them both to avoid breaking anything. > > > [1] Actually, until 972fc544b6034a in uq/master is merged there won't be > > any warnings either. > > > > Signed-off-by: Andrew Jones <drjones@xxxxxxxxxx> > > --- > > arch/x86/include/asm/kvm_host.h | 1 - > > arch/x86/kvm/x86.c | 2 +- > > 2 files changed, 1 insertion(+), 2 deletions(-) > > > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h > > index c76ff74a98f2e..9236c63315a9b 100644 > > --- a/arch/x86/include/asm/kvm_host.h > > +++ b/arch/x86/include/asm/kvm_host.h > > @@ -32,7 +32,6 @@ > > #include <asm/asm.h> > > > > #define KVM_MAX_VCPUS 255 > > -#define KVM_SOFT_MAX_VCPUS 160 > > #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/x86.c b/arch/x86/kvm/x86.c > > index e5ca72a5cdb6d..d9d3e2ed68ee9 100644 > > --- a/arch/x86/kvm/x86.c > > +++ b/arch/x86/kvm/x86.c > > @@ -2604,7 +2604,7 @@ int kvm_dev_ioctl_check_extension(long ext) > > r = !kvm_x86_ops->cpu_has_accelerated_tpr(); > > break; > > case KVM_CAP_NR_VCPUS: > > - r = KVM_SOFT_MAX_VCPUS; > > + r = min(num_online_cpus(), KVM_MAX_VCPUS); > s/KVM_MAX_VCPUS/KVM_SOFT_MAX_VCPUS/. Also what about hotplug cpus? I'll send a v2 with this change. I thought a bit about hotplug, and thus using num_possible_cpus() instead, but then decided it made more sense to stick to what's online now for the recommended number. It's just a recommendation anyway. So as long as KVM_MAX_VCPUS is >= num_possible_cpus(), then one can still configure more vcpus to count for all hotplugable cpus, if they wish. drew -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
