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Subject[PATCH 6/6] x86/kvm: add boot parameter for setting max number of vcpus per guest
Date
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 | 10 ++++++++++
arch/x86/include/asm/kvm_host.h | 5 ++++-
arch/x86/kvm/x86.c | 7 +++++++
3 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 99bfa53a2bbd..8eb856396ffa 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2373,6 +2373,16 @@
guest can't have more vcpus than the set value + 1.
Default: 1023

+ 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. Depending
+ on the value and the virtual topology the maximum
+ allowed vcpu-id might need to be raised, too (see
+ kvm.max_vcpu_id parameter).
+ Default: 288
+
l1tf= [X86] Control mitigation of the L1TF vulnerability on
affected CPUs

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 39cbc4b6bffb..65ae82a5d444 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -37,7 +37,8 @@

#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_DEFAULT_MAX_VCPU_ID 1023
#define KVM_MAX_VCPU_ID max_vcpu_id
@@ -1509,6 +1510,8 @@ 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 */
extern unsigned int max_vcpu_id;
/* per cpu vcpu bitmasks (disable preemption during usage) */
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index a9b0bb2221ea..888c4507504d 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -177,6 +177,10 @@ module_param(force_emulation_prefix, bool, S_IRUGO);
int __read_mostly pi_inject_timer = -1;
module_param(pi_inject_timer, bint, S_IRUGO | S_IWUSR);

+unsigned int __read_mostly max_vcpus = KVM_DEFAULT_MAX_VCPUS;
+module_param(max_vcpus, uint, S_IRUGO);
+EXPORT_SYMBOL_GPL(max_vcpus);
+
unsigned int __read_mostly max_vcpu_id = KVM_DEFAULT_MAX_VCPU_ID;
module_param(max_vcpu_id, uint, S_IRUGO);

@@ -10648,6 +10652,9 @@ 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
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