Eduardo Habkost <ehabkost@xxxxxxxxxx> writes: > On Fri, Sep 03, 2021 at 03:08:04PM +0200, Juergen Gross wrote: >> In order to support high vcpu numbers per guest don't use on stack >> vcpu bitmasks. As all those currently used bitmasks are not used in >> functions subject to recursion it is fairly easy to replace them with >> percpu bitmasks. >> >> Disable preemption while such a bitmask is being used in order to >> avoid double usage in case we'd switch cpus. >> >> Signed-off-by: Juergen Gross <jgross@xxxxxxxx> >> --- >> V2: >> - use local_lock() instead of preempt_disable() (Paolo Bonzini) >> --- >> arch/x86/include/asm/kvm_host.h | 10 ++++++++++ >> arch/x86/kvm/hyperv.c | 25 ++++++++++++++++++------- >> arch/x86/kvm/irq_comm.c | 9 +++++++-- >> arch/x86/kvm/x86.c | 22 +++++++++++++++++++++- >> 4 files changed, 56 insertions(+), 10 deletions(-) >> >> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h >> index 3513edee8e22..a809a9e4fa5c 100644 >> --- a/arch/x86/include/asm/kvm_host.h >> +++ b/arch/x86/include/asm/kvm_host.h >> @@ -15,6 +15,7 @@ >> #include <linux/cpumask.h> >> #include <linux/irq_work.h> >> #include <linux/irq.h> >> +#include <linux/local_lock.h> >> >> #include <linux/kvm.h> >> #include <linux/kvm_para.h> >> @@ -1591,6 +1592,15 @@ extern bool kvm_has_bus_lock_exit; >> /* maximum vcpu-id */ >> unsigned int kvm_max_vcpu_id(void); >> >> +/* per cpu vcpu bitmasks, protected by kvm_pcpu_mask_lock */ >> +DECLARE_PER_CPU(local_lock_t, kvm_pcpu_mask_lock); >> +extern unsigned long __percpu *kvm_pcpu_vcpu_mask; >> +#define KVM_VCPU_MASK_SZ \ >> + (sizeof(*kvm_pcpu_vcpu_mask) * BITS_TO_LONGS(KVM_MAX_VCPUS)) >> +extern u64 __percpu *kvm_hv_vp_bitmap; >> +#define KVM_HV_MAX_SPARSE_VCPU_SET_BITS DIV_ROUND_UP(KVM_MAX_VCPUS, 64) >> +#define KVM_HV_VPMAP_SZ (sizeof(u64) * KVM_HV_MAX_SPARSE_VCPU_SET_BITS) > > I have just realized that the Hyper-V sparse bitmap format can > support only up to 4096 CPUs, and the current implementation of > sparse_set_to_vcpu_mask() won't even work correctly if > KVM_MAX_VCPUS is larger than 4096. Nice catch! Indeed, we can only encode 64 'banks' with 64 vCPUs each. We need to enforce the limit somehow. As a big hammer, I can suggest to fail kvm_hv_vcpu_init() and write to HV_X64_MSR_VP_INDEX for vCPUs above 4095 for now (I seriously doubt there's real need to run such big Windows guests anywhere but who knows). > > This means vp_bitmap can't and will never be larger than 512 > bytes. Isn't a per-CPU variable for vp_bitmap overkill in this > case? Yes, it's OK to allocate 512 bytes on stack. -- Vitaly
