refactor: Calc right-shift bits for socket domain

The number of bits that the x2APIC ID must be shifted to the right to
address instances of the socket domain, which is next higher-scoped
domain to the core domain, was hardcoded as 7. That means up to 128
vcpus support. Currently the max vcpu count is hardcoded as 32, so it
is enough but we might want to increase it in the future. To avoid
unexpected issues when increasing the max vcpu count, calculate the
value based on the max vCPU count.

Signed-off-by: Takahiro Itazuri <[email protected]>

Author: zulinx86

src/vmm/src/cpu_config/x86_64/cpuid/normalize.rs

@@ -282,11 +282,6 @@ impl super::Cpuid {
         cpu_bits: u8,
         cpus_per_core: u8,
     ) -> Result<(), ExtendedTopologyError> {
-        /// The APIC ID shift in leaf 0xBh specifies the number of bits to shit the x2APIC ID to
-        /// get a unique topology of the next level. This allows 128 logical
-        /// processors/package.
-        const LEAFBH_INDEX1_APICID: u32 = 7;
-
         // The following commit changed the behavior of KVM_GET_SUPPORTED_CPUID to no longer
         // include CPUID.(EAX=0BH,ECX=1).
         // https://lore.kernel.org/all/[email protected]/
@@ -365,11 +360,19 @@ impl super::Cpuid {
                     }
                     // Core domain
                     1 => {
-                        set_range(&mut subleaf.result.eax, 0..5, LEAFBH_INDEX1_APICID)
-                            .map_err(ExtendedTopologyError::ApicId)?;
-
                         // Configure such that the next higher-scoped domain (i.e. socket) include
                         // all logical processors.
+                        //
+                        // The CPUID.(EAX=0BH,ECX=1).EAX[4:0] value should be greater than or equal
+                        // to the base 2 logarithm of the number of logical processors, rounded up
+                        // to the decimal point. Since the maximum number of vCPUs is hardcoded and
+                        // is known a factorial of 2, use ilog2() of it.
+                        set_range(
+                            &mut subleaf.result.eax,
+                            0..5,
+                            crate::vmm_config::machine_config::MAX_SUPPORTED_VCPUS.ilog2(),
+                        )
+                        .map_err(ExtendedTopologyError::ApicId)?;
                         set_range(&mut subleaf.result.ebx, 0..16, u32::from(cpu_count))
                             .map_err(ExtendedTopologyError::LogicalProcessors)?;