Commit f10a570b authored by Kyle Meyer's avatar Kyle Meyer Committed by Sean Christopherson
Browse files

KVM: x86: Add CONFIG_KVM_MAX_NR_VCPUS to allow up to 4096 vCPUs 



Add a Kconfig entry to set the maximum number of vCPUs per KVM guest and
set the default value to 4096 when MAXSMP is enabled, as there are use
cases that want to create more than the currently allowed 1024 vCPUs and
are more than happy to eat the memory overhead.

The Hyper-V TLFS doesn't allow more than 64 sparse banks, i.e. allows a
maximum of 4096 virtual CPUs. Cap KVM's maximum number of virtual CPUs
to 4096 to avoid exceeding Hyper-V's limit as KVM support for Hyper-V is
unconditional, and alternatives like dynamically disabling Hyper-V
enlightenments that rely on sparse banks would require non-trivial code
changes.

Suggested-by: default avatarSean Christopherson <seanjc@google.com>
Signed-off-by: default avatarKyle Meyer <kyle.meyer@hpe.com>
Link: https://lore.kernel.org/r/20230824215244.3897419-1-kyle.meyer@hpe.com


[sean: massage changelog with --verbose, document #ifdef mess]
Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
parent 4346db6e

arch/x86/include/asm/kvm_host.h

+8 −0
Original line number Diff line number Diff line
@@ -39,7 +39,15 @@ 


#define __KVM_HAVE_ARCH_VCPU_DEBUGFS



/*

 * CONFIG_KVM_MAX_NR_VCPUS is defined iff CONFIG_KVM!=n, provide a dummy max if

 * KVM is disabled (arbitrarily use the default from CONFIG_KVM_MAX_NR_VCPUS).

 */

#ifdef CONFIG_KVM_MAX_NR_VCPUS

#define KVM_MAX_VCPUS CONFIG_KVM_MAX_NR_VCPUS

#else

#define KVM_MAX_VCPUS 1024

#endif



/*

 * In x86, the VCPU ID corresponds to the APIC ID, and APIC IDs

arch/x86/kvm/Kconfig

+11 −0
Original line number Diff line number Diff line
@@ -154,4 +154,15 @@ config KVM_PROVE_MMU 
config KVM_EXTERNAL_WRITE_TRACKING

	bool



config KVM_MAX_NR_VCPUS

	int "Maximum number of vCPUs per KVM guest"

	depends on KVM

	range 1024 4096

	default 4096 if MAXSMP

	default 1024

	help

	  Set the maximum number of vCPUs per KVM guest. Larger values will increase

	  the memory footprint of each KVM guest, regardless of how many vCPUs are

	  created for a given VM.



endif # VIRTUALIZATION