Re: [PATCH v2 6/6] x86/kvm: add boot parameter for setting max number of vcpus per guest

[Yao Yuan <yaoyuan0329os@gmail.com>]

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@suse.com>
> ---
>  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?

>
>   if (vcpu_id_add_bits < -1 || vcpu_id_add_bits > (32 - n_bits)) {
>       pr_err("Invalid value of vcpu_id_add_bits=%d parameter!\n",
> @@ -11033,6 +11037,9 @@ int kvm_arch_hardware_setup(void *opaque)
>   if (boot_cpu_has(X86_FEATURE_XSAVES))
>       rdmsrl(MSR_IA32_XSS, host_xss);
>
> +	if (max_vcpus == 0)
> +		max_vcpus = num_possible_cpus();
> +
>   kvm_pcpu_vcpu_mask = __alloc_percpu(KVM_VCPU_MASK_SZ,
>                       sizeof(unsigned long));
>   kvm_hv_vp_bitmap = __alloc_percpu(KVM_HV_VPMAP_SZ, sizeof(u64));
> --
> 2.26.2
>