linux-kernel - Re: x86/fpu: Inaccurate AVX-512 Usage Tracking via arch

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CACueBy4MTDesg_icWRYrAEJxZ3m3_Qo-joQUdgV0tqRWFe8Y4A@mail.gmail.com>
Date: Thu, 30 Oct 2025 14:56:09 +0800
From: chuang <nashuiliang@...il.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, open list <linux-kernel@...r.kernel.org>
Subject: Re: x86/fpu: Inaccurate AVX-512 Usage Tracking via arch_status

On Mon, Oct 27, 2025 at 10:26 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 10/27/25 00:50, chuang wrote:
> > On AVX-512 capable systems, the implementation appears to record the
> > current timestamp into 'task->thread.fpu.avx512_timestamp' upon any
> > task switch, irrespective of whether the task has actually executed an
> > AVX-512 instruction.
>
> The timestamp update ultimately has _zero_ to do with executing
> AVX-512 instructions. It's all about the state in the ZMM registers, not
> AVX-512 instructions.

Got it, thanks.

> Those registers are inherited at fork and I don't see avx512_timestamp
> being zeroed anywhere. So I suspect what you are seeing is that some
> _parent_ used AVX512, and its children are getting stuck with
> avx512_timestamp.

I have tested with the attached patch. The behavior remains the same
as previously reported: after fpu_clone(), the new process still has a
non-zero avx512_timestamp, and it continues to be updated in
subsequent task switches, irrespective of AVX-512 instruction
execution.

I traced the code path within fpu_clone(): In fpu_clone() ->
save_fpregs_to_fpstate(), since my current Intel CPU supports XSAVE,
the call to os_xsave() results in the XFEATURE_Hi16_ZMM bit being
set/enabled in xsave.header.xfeatures. This then causes
update_avx_timestamp() to update fpu->avx512_timestamp. The same flow
occurs in __switch_to() -> switch_fpu_prepare().

Given this, is the issue related to my specific Intel Xeon Gold? Is
the CPU continuously indicating that the AVX-512 state is in use?

> You could probably confirm this by dumping ->avx512_timestamp in
> fpu_clone().
>
> Or, try the attached patch and see if it makes things work more like
> you'd expect.

Best regards,