On Mon, 2018-09-10 at 10:10 -0500, Brijesh Singh wrote: > > On 09/10/2018 08:29 AM, Sean Christopherson wrote: > ... > > > > > > + */ > > > > > +static struct pvclock_vsyscall_time_info > > > > > + hv_clock_aux[NR_CPUS] __decrypted_aux; > > > > Hmm, so worst case that's 64 4K pages: > > > > > > > > (8192*32)/4096 = 64 4K pages. > > > We can minimize the worst case memory usage. The number of VCPUs > > > supported by KVM maybe less than NR_CPUS. e.g Currently KVM_MAX_VCPUS is > > > set to 288 > > KVM_MAX_VCPUS is a property of the host, whereas this code runs in the > > guest, e.g. KVM_MAX_VCPUS could be 2048 in the host for all we know. > > > > IIRC, during guest creation time qemu will check the host supported > VCPUS count. If count is greater than KVM_MAX_VCPUS then it will > fail to launch guest (or fail to hot plug vcpus). In other words, the > number of vcpus in a KVM guest will never to > KVM_MAX_VCPUS. > > Am I missing something ? KVM_MAX_VCPUS is a definition for use in the *host*, it's even defined in kvm_host.h. The guest's pvclock code won't get magically recompiled if KVM_MAX_VCPUS is changed in the host. KVM_MAX_VCPUS is an arbitrary value in the sense that there isn't a fundamental hard limit, i.e. the value can be changed, either for a custom KVM build or in mainline, e.g. it was bumped in 2016: commit 682f732ecf7396e9d6fe24d44738966699fae6c0 Author: Radim Krčmář <rkrcmar@xxxxxxxxxx> Date: Tue Jul 12 22:09:29 2016 +0200 KVM: x86: bump MAX_VCPUS to 288 288 is in high demand because of Knights Landing CPU. We cannot set the limit to 640k, because that would be wasting space. Reviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> Signed-off-by: Radim Krčmář <rkrcmar@xxxxxxxxxx> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 074b5c760327..21a40dc7aad6 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -34,7 +34,7 @@ #include <asm/asm.h> #include <asm/kvm_page_track.h> -#define KVM_MAX_VCPUS 255 +#define KVM_MAX_VCPUS 288 #define KVM_SOFT_MAX_VCPUS 240 #define KVM_USER_MEM_SLOTS 509 /* memory slots that are not exposed to userspace */
