arch/x86_64: fix cpuid leaf 0x1 EBX bits 23-16#7485
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rbradford merged 1 commit intocloud-hypervisor:mainfrom Nov 18, 2025
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arch/x86_64: fix cpuid leaf 0x1 EBX bits 23-16#7485rbradford merged 1 commit intocloud-hypervisor:mainfrom
rbradford merged 1 commit intocloud-hypervisor:mainfrom
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Commit 5ec47d4 was intended to patch ebx bits 23-16 in cpuid leaf 0x1, but it was not working as expected, as in rust, operator << has a stronger precedence than & [1]. Later commit b6667f9 fixed the operator precedence clippy warning, but did not fix the actual issue. As a result, the current code is not changing ebx, ``` cpu_ebx |= ((dies_per_package as u32) * (cores_per_die as u32) * (threads_per_core as u32)) & (0xff << 16); ``` Since the total number of logical processors is generally less than 65536, the right hand side of the expression is 0 in most cases. [1] https://doc.rust-lang.org/reference/expressions.html#expression-precedence Fixes: 5ec47d4 ("arch: x86_64: enable HTT flag") Signed-off-by: Changyuan Lyu <changyuanl@google.com>
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Ping @olivereanderson as he is currently working on cpu profiles for #7068 and is currently an expert for all the cpu leaves. Perhaps one of the bugs you are seeing is caused by this, Oliver? |
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rbradford
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Nov 17, 2025
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Hmm, could be, but we should be seeing problems with this even outside of my branch. |
phip1611
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Commit 5ec47d4 was intended to patch ebx bits 23-16 in cpuid leaf 0x1, but it was not working as expected, as in rust, operator << has a stronger precedence than & [1]. Later commit b6667f9 fixed the operator precedence clippy warning, but did not fix the actual issue. As a result, the current code is not changing ebx,
Since the total number of logical processors is generally less than 65536, the right hand side of the expression is 0 in most cases.
[1] https://doc.rust-lang.org/reference/expressions.html#expression-precedence
Fixes: 5ec47d4 ("arch: x86_64: enable HTT flag")