perf(config): optimize the x86 guest kernel (#25)

* fix(kernel): update Linux to 6.12.99

Advance the bundled stable kernel by one patch release so the firmware
build remains on the current Linux 6.12.y baseline.

Keep the existing libkrunfw version, ABI, reproducible timestamp, and
patch stack unchanged.

* perf(config): accelerate x86 CRC32C

Build the kernel's feature-detected CRC32C implementation so capable AMD and Intel guests can use hardware acceleration for ext4 metadata checksums instead of crc32c-generic.

* perf(config): make x86 preemption mode static

Disable dynamic preemption in the x86 firmware configuration so the
PREEMPT_NONE selection is compiled as the sole scheduling model.

Keep this change isolated for hardlink throughput and responsiveness
A/B testing before it is considered for the cumulative stack.

* perf(config): enable x2APIC for interrupt benchmarks

Enable the x86 guest kernel's x2APIC support on top of the cumulative
CRC32C and static PREEMPT_NONE firmware. Keep the change isolated so its
interrupt-routing and workload effects can be accepted or rejected
without changing the cumulative performance branch.

Author: appcypher

Makefile

@@ -1,4 +1,4 @@
-KERNEL_VERSION = linux-6.12.98
+KERNEL_VERSION = linux-6.12.99
 # Kernel source from the kernel.org CDN (static release tarball).
 KERNEL_REMOTE = https://cdn.kernel.org/pub/linux/kernel/v6.x/$(KERNEL_VERSION).tar.gz
 KERNEL_TARBALL = tarballs/$(KERNEL_VERSION).tar.gz

config-libkrunfw_x86_64

@@ -128,7 +128,7 @@ CONFIG_PREEMPT_NONE=y
 # CONFIG_PREEMPT_RT is not set
 CONFIG_PREEMPT_COUNT=y
 CONFIG_PREEMPTION=y
-CONFIG_PREEMPT_DYNAMIC=y
+# CONFIG_PREEMPT_DYNAMIC is not set
 # CONFIG_SCHED_CORE is not set
 
 #
@@ -319,7 +319,7 @@ CONFIG_CC_HAS_SANE_STACKPROTECTOR=y
 # Processor type and features
 #
 CONFIG_SMP=y
-# CONFIG_X86_X2APIC is not set
+CONFIG_X86_X2APIC=y
 CONFIG_X86_MPPARSE=y
 # CONFIG_X86_CPU_RESCTRL is not set
 # CONFIG_X86_FRED is not set
@@ -2666,7 +2666,7 @@ CONFIG_CRYPTO_KDF800108_CTR=y
 # CONFIG_CRYPTO_SHA512_SSSE3 is not set
 # CONFIG_CRYPTO_SM3_AVX_X86_64 is not set
 # CONFIG_CRYPTO_GHASH_CLMUL_NI_INTEL is not set
-# CONFIG_CRYPTO_CRC32C_INTEL is not set
+CONFIG_CRYPTO_CRC32C_INTEL=y
 # CONFIG_CRYPTO_CRC32_PCLMUL is not set
 CONFIG_CRYPTO_CRCT10DIF_PCLMUL=y
 # end of Accelerated Cryptographic Algorithms for CPU (x86)