arch: x86_64: Use host cpuid information for L2 cache for older KVM

If the KVM version is too old (pre Linux 5.7) then fetch the CPUID
information from the host and use that in the guest. We prefer the KVM
version over the host version as that would use the CPUID for the
running CPU vs the CPU that runs this code which might be different due
to a hybrid topology.

Fixes: #4918

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

arch/src/x86_64/mod.rs

@@ -633,6 +633,20 @@ pub fn generate_common_cpuid(
                     }
                 }
             }
+            // Copy host L2 cache details if not populated by KVM
+            0x8000_0006 => {
+                if entry.eax == 0 && entry.ebx == 0 && entry.ecx == 0 && entry.edx == 0 {
+                    // SAFETY: cpuid called with valid leaves
+                    if unsafe { std::arch::x86_64::__cpuid(0x8000_0000).eax } >= 0x8000_0006 {
+                        // SAFETY: cpuid called with valid leaves
+                        let leaf = unsafe { std::arch::x86_64::__cpuid(0x8000_0006) };
+                        entry.eax = leaf.eax;
+                        entry.ebx = leaf.ebx;
+                        entry.ecx = leaf.ecx;
+                        entry.edx = leaf.edx;
+                    }
+                }
+            }
             // Set CPU physical bits
             0x8000_0008 => {
                 entry.eax = (entry.eax & 0xffff_ff00) | (phys_bits as u32 & 0xff);