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?
Attachment:
OpenPGP_0xB0DE9DD628BF132F.asc
Description: OpenPGP public key
Attachment:
OpenPGP_signature
Description: OpenPGP digital signature