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.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Kyle Meyer <kyle.meyer@hpe.com>
---
 arch/x86/include/asm/kvm_host.h |  2 +-
 arch/x86/kvm/Kconfig            | 11 +++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 3bc146dfd38d..536317812b52 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -39,7 +39,7 @@
 
 #define __KVM_HAVE_ARCH_VCPU_DEBUGFS
 
-#define KVM_MAX_VCPUS 1024
+#define KVM_MAX_VCPUS CONFIG_KVM_MAX_NR_VCPUS
 
 /*
  * 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.
+
 endif # VIRTUALIZATION
-- 
2.26.2

Hi Kyle,

kernel test robot noticed the following build errors:

[auto build test ERROR on kvm/queue]
[also build test ERROR on linus/master v6.5-rc7 next-20230823]
[cannot apply to kvm/linux-next]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Kyle-Meyer/KVM-x86-Add-CONFIG_KVM_MAX_NR_VCPUS/20230824-034224
base:   https://git.kernel.org/pub/scm/virt/kvm/kvm.git queue
patch link:    https://lore.kernel.org/r/20230823193842.2544394-1-kyle.meyer%40hpe.com
patch subject: [PATCH v2] KVM: x86: Add CONFIG_KVM_MAX_NR_VCPUS
config: i386-tinyconfig (https://download.01.org/0day-ci/archive/20230824/202308240540.9fQYjHB2-lkp@intel.com/config)
compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
reproduce: (https://download.01.org/0day-ci/archive/20230824/202308240540.9fQYjHB2-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202308240540.9fQYjHB2-lkp@intel.com/

All errors (new ones prefixed by >>):

   In file included from include/vdso/const.h:5,
                    from include/linux/const.h:4,
                    from include/uapi/linux/kernel.h:6,
                    from include/linux/cache.h:5,
                    from include/linux/slab.h:15,
                    from arch/x86/events/intel/core.c:14:
>> arch/x86/include/asm/kvm_host.h:42:23: error: 'CONFIG_KVM_MAX_NR_VCPUS' undeclared here (not in a function)
      42 | #define KVM_MAX_VCPUS CONFIG_KVM_MAX_NR_VCPUS
         |                       ^~~~~~~~~~~~~~~~~~~~~~~
   include/uapi/linux/const.h:34:40: note: in definition of macro '__KERNEL_DIV_ROUND_UP'
      34 | #define __KERNEL_DIV_ROUND_UP(n, d) (((n) + (d) - 1) / (d))
         |                                        ^
   arch/x86/include/asm/kvm_host.h:1120:33: note: in expansion of macro 'BITS_TO_LONGS'
    1120 |         unsigned long poll_mask[BITS_TO_LONGS(KVM_MAX_VCPUS)];
         |                                 ^~~~~~~~~~~~~
   arch/x86/include/asm/kvm_host.h:1120:47: note: in expansion of macro 'KVM_MAX_VCPUS'
    1120 |         unsigned long poll_mask[BITS_TO_LONGS(KVM_MAX_VCPUS)];
         |                                               ^~~~~~~~~~~~~


vim +/CONFIG_KVM_MAX_NR_VCPUS +42 arch/x86/include/asm/kvm_host.h

    41	
  > 42	#define KVM_MAX_VCPUS CONFIG_KVM_MAX_NR_VCPUS
    43	

-- 
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https://github.com/intel/lkp-tests/wiki