git: DragonFly_RELEASE_6_4 cpu/x86_64: Fix do_cpuid() to explicitly set ECX=0

[Aaron LI]

commit 6582506c4b126170053d230aeec9dd800b3af9a3
Author: Aaron LI <aly at aaronly.me>
Date:   Mon Jan 26 13:03:04 2026 +0800

    cpu/x86_64: Fix do_cpuid() to explicitly set ECX=0
    
    The old do_cpuid() did not initialize ECX before executing the CPUID
    instruction, so the results could be incorrect when ECX contained a
    non-zero garbage value.
    
    This issue was observed on Intel CPUs when booting a FreeBSD 14.x/15.x
    ISO under NVMM, where it caused a general protection fault (#GP) shortly
    after the FreeBSD kernel was loaded:
    
        qemu-system-x86_64: NVMM: Mem Assist Failed [gpa=0xbfff8]
        qemu-system-x86_64: NVMM: Failed to execute a VCPU.
        Abort trap (core dumped)
    
    It occurred when NVMM tried to handle the reading of
    IA32_ARCH_CAPABILITIES MSR but the second do_cpuid() returned the
    incorrect results indicating that the MSR was unavailable.
    
    The problem was first reported by mneumann in bug #3310 on 2025-11-26 [1].
    
    With this fix applied, NVMM successfully boots FreeBSD 15.0 on my Intel
    i5-1340P system.
    
    The root cause was identified by tuxillo with the assistance of the
    Claude Opus 4.5 LLM.
    
    [1] https://bugs.dragonflybsd.org/issues/3310#note-9
    
    Credit: Claude Opus 4.5
    Wizard: tuxillo

Summary of changes:
 sys/cpu/x86_64/include/cpufunc.h | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

http://gitweb.dragonflybsd.org/dragonfly.git/commitdiff/6582506c4b126170053d230aeec9dd800b3af9a3


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