On Tue, Nov 16, 2021, Juergen Gross wrote: > When emulating Hyperv the theoretical maximum of vcpus supported is > 4096, as this is the architectural limit for sending IPIs via the PV > interface. > > For restricting the actual supported number of vcpus for that case > introduce another define KVM_MAX_HYPERV_VCPUS and set it to 1024, like > today's KVM_MAX_VCPUS. Make both values unsigned ones as this will be > needed later. > > The actual number of supported vcpus for Hyperv emulation will be the > lower value of both defines. > > This is a preparation for a future boot parameter support of the max > number of vcpus for a KVM guest. > > Signed-off-by: Juergen Gross <jgross@xxxxxxxx> > --- > V3: > - new patch > --- > arch/x86/include/asm/kvm_host.h | 3 ++- > arch/x86/kvm/hyperv.c | 15 ++++++++------- > 2 files changed, 10 insertions(+), 8 deletions(-) > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h > index 886930ec8264..8ea03ff01c45 100644 > --- a/arch/x86/include/asm/kvm_host.h > +++ b/arch/x86/include/asm/kvm_host.h > @@ -38,7 +38,8 @@ > > #define __KVM_HAVE_ARCH_VCPU_DEBUGFS > > -#define KVM_MAX_VCPUS 1024 > +#define KVM_MAX_VCPUS 1024U > +#define KVM_MAX_HYPERV_VCPUS 1024U I don't see any reason to put this in kvm_host.h, it should never be used outside of hyperv.c. > #define KVM_MAX_VCPU_IDS kvm_max_vcpu_ids() > /* memory slots that are not exposed to userspace */ > #define KVM_PRIVATE_MEM_SLOTS 3 > diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c > index 4a555f32885a..c0fa837121f1 100644 > --- a/arch/x86/kvm/hyperv.c > +++ b/arch/x86/kvm/hyperv.c > @@ -41,7 +41,7 @@ > /* "Hv#1" signature */ > #define HYPERV_CPUID_SIGNATURE_EAX 0x31237648 > > -#define KVM_HV_MAX_SPARSE_VCPU_SET_BITS DIV_ROUND_UP(KVM_MAX_VCPUS, 64) > +#define KVM_HV_MAX_SPARSE_VCPU_SET_BITS DIV_ROUND_UP(KVM_MAX_HYPERV_VCPUS, 64) > > static void stimer_mark_pending(struct kvm_vcpu_hv_stimer *stimer, > bool vcpu_kick); > @@ -166,7 +166,7 @@ static struct kvm_vcpu *get_vcpu_by_vpidx(struct kvm *kvm, u32 vpidx) > struct kvm_vcpu *vcpu = NULL; > int i; > > - if (vpidx >= KVM_MAX_VCPUS) > + if (vpidx >= min(KVM_MAX_VCPUS, KVM_MAX_HYPERV_VCPUS)) IMO, this is conceptually wrong. KVM should refuse to allow Hyper-V to be enabled if the max number of vCPUs exceeds what can be supported, or should refuse to create the vCPUs. I agree it makes sense to add a Hyper-V specific limit, since there are Hyper-V structures that have a hard limit, but detection of violations should be a BUILD_BUG_ON, not a silent failure at runtime.
