On 06.09.21 02:45, Yao Yuan wrote:
On Fri, Sep 03, 2021 at 03:08:07PM +0200, Juergen Gross wrote:Today the maximum number of vcpus of a kvm guest is set via a #define in a header file. In order to support higher vcpu numbers for guests without generally increasing the memory consumption of guests on the host especially on very large systems add a boot parameter for specifying the number of allowed vcpus for guests. The default will still be the current setting of 288. The value 0 has the special meaning to limit the number of possible vcpus to the number of possible cpus of the host. Signed-off-by: Juergen Gross <jgross@xxxxxxxx> --- Documentation/admin-guide/kernel-parameters.txt | 7 +++++++ arch/x86/include/asm/kvm_host.h | 5 ++++- arch/x86/kvm/x86.c | 9 ++++++++- 3 files changed, 19 insertions(+), 2 deletions(-) diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 37e194299311..b9641c9989ef 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -2435,6 +2435,13 @@ feature (tagged TLBs) on capable Intel chips. Default is 1 (enabled) + kvm.max_vcpus= [KVM,X86] Set the maximum allowed numbers of vcpus per + guest. The special value 0 sets the limit to the number + of physical cpus possible on the host (including not + yet hotplugged cpus). Higher values will result in + slightly higher memory consumption per guest. + Default: 288 + kvm.vcpu_id_add_bits= [KVM,X86] The vcpu-ids of guests are sparse, as they are constructed by bit-wise concatenation of the ids of diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 6c28d0800208..a4ab387b0e1c 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 288 +#define KVM_DEFAULT_MAX_VCPUS 288 +#define KVM_MAX_VCPUS max_vcpus #define KVM_SOFT_MAX_VCPUS 240 #define KVM_MAX_VCPU_ID kvm_max_vcpu_id() /* memory slots that are not exposed to userspace */ @@ -1588,6 +1589,8 @@ extern u64 kvm_max_tsc_scaling_ratio; extern u64 kvm_default_tsc_scaling_ratio; /* bus lock detection supported? */ extern bool kvm_has_bus_lock_exit; +/* maximum number of vcpus per guest */ +extern unsigned int max_vcpus; /* maximum vcpu-id */ unsigned int kvm_max_vcpu_id(void); diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index ff142b6dd00c..49c3d91c559e 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -188,9 +188,13 @@ module_param(pi_inject_timer, bint, S_IRUGO | S_IWUSR); static int __read_mostly vcpu_id_add_bits = -1; module_param(vcpu_id_add_bits, int, S_IRUGO); +unsigned int __read_mostly max_vcpus = KVM_DEFAULT_MAX_VCPUS; +module_param(max_vcpus, uint, S_IRUGO); +EXPORT_SYMBOL_GPL(max_vcpus); + unsigned int kvm_max_vcpu_id(void) { - int n_bits = fls(KVM_MAX_VCPUS - 1); + int n_bits = fls(max_vcpus - 1);A quesintion here: the parameter "vcpu_id_add_bits" also depends on the "max_vcpus", we can't calculate the "vcpu_id_add_bits" from "max_vcpus" because KVM has no topologically knowledge to determine bits needed for each socket/core/thread level, right?
Correct. Juergen
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