On 4/29/25 11:02 AM, Ewan Hai wrote:
Hi Community,
This email contains 3 bugs appear to share the same root cause.
[1] We ran into the following warnings when running QEMU v10.0.0 in TCG mode:
qemu-system-x86_64 \
-machine q35 \
-m 4G -smp 4 \
-kernel ./arch/x86/boot/bzImage \
-bios /usr/share/ovmf/OVMF.fd \
-drive file=~/kernel/rootfs.ext4,index=0,format=raw,media=disk \
-drive file=~/kernel/swap.img,index=1,format=raw,media=disk \
-nographic \
-append 'root=/dev/sda rw resume=/dev/sdb console=ttyS0 nokaslr'
qemu-system-x86_64: warning: TCG doesn't support requested feature:
CPUID.01H:EDX.ht [bit 28]
qemu-system-x86_64: warning: TCG doesn't support requested feature:
CPUID.80000001H:ECX.cmp-legacy [bit 1]
(repeats 4 times, once per vCPU)
Tracing the history shows that commit c6bd2dd63420 "i386/cpu: Set up CPUID_HT in
x86_cpu_expand_features() instead of cpu_x86_cpuid()" is what introduced the
warnings.
Since that commit, TCG unconditionally advertises HTT (CPUID 1 EDX[28]) and
CMP_LEG (CPUID 8000_0001 ECX[1]). Because TCG itself has no SMT support, these
bits trigger the warnings above.
[2] Also, Zhao pointed me to a similar report on GitLab:
https://gitlab.com/qemu-project/qemu/-/issues/2894
The symptoms there look identical to what we're seeing.
By convention we file one issue per email, but these two appear to share the
same root cause, so I'm describing them together here.
[3] My colleague Alan noticed what appears to be a related problem: if we launch
a guest with '-cpu <model>,-ht --enable-kvm', which means explicitly removing
the ht flag, but the guest still reports HT(cat /proc/cpuinfo in linux guest)
enabled. In other words, under KVM the ht bit seems to be forced on even when
the user tries to disable it.
XiaoYao reminded me that issue [3] stems from a different patch. Please ignore
it for now—I'll start a separate thread to discuss that one independently.
Best regards,
Ewan
