> > 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. > > I'd like to capture why the max is set to 4096, both the justification and why > we don't want to go further at this point. > > If you've no objection, I'll massage the changelog to this when applying: > > 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 allow 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: Sean Christopherson <seanjc@xxxxxxxxxx> > > Signed-off-by: Kyle Meyer <kyle.meyer@xxxxxxx> > > --- > > v2 -> v3: Default KVM_MAX_VCPUS to 1024 when CONFIG_KVM_MAX_NR_VCPUS is not > > defined. This prevents build failures in arch/x86/events/intel/core.c and > > drivers/vfio/vfio_main.c when KVM is disabled. > > > > arch/x86/include/asm/kvm_host.h | 4 ++++ > > arch/x86/kvm/Kconfig | 11 +++++++++++ > > 2 files changed, 15 insertions(+) > > > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h > > index 3bc146dfd38d..cd27e0a00765 100644 > > --- a/arch/x86/include/asm/kvm_host.h > > +++ b/arch/x86/include/asm/kvm_host.h > > @@ -39,7 +39,11 @@ > > > > #define __KVM_HAVE_ARCH_VCPU_DEBUGFS > > > > And another thing I'll add if you don't object is a comment to explain that this > is purely to play nice with CONFIG_KVM=n. And FWIW, I hope to make this go away > entirely: https://lore.kernel.org/all/20230916003118.2540661-27-seanjc@xxxxxxxxxx > > /* > * CONFIG_KVM_MAX_NR_VCPUS is defined iff CONFIG_KVM!=n, provide a dummy max if > * KVM is disabled (arbitrarily use 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 > > diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig > > index 89ca7f4c1464..e730e8255e22 100644 > > --- a/arch/x86/kvm/Kconfig > > +++ b/arch/x86/kvm/Kconfig > > @@ -141,4 +141,15 @@ config KVM_XEN > > 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 > > + configured. > > Last nit, I think the last linke should be like so: > > the memory footprint of each KVM guest, regardless of how many vCPUs are > created for a given VM. > > No need for a v4 unless you object to any of the above, I'm happt to fixup when > applying. LGTM. I have no objections to any of those changes. Thank you very much.
