Re: [PATCH v3] x86: fpu: Keep xfd_state always in sync with MSR_IA32_XFD

[Chang S. Bae]

On 5/19/2023 4:23 AM, Adamos Ttofari wrote:
> Commit 672365477ae8 ("x86/fpu: Update XFD state where required") and
> commit 8bf26758ca96 ("x86/fpu: Add XFD state to fpstate") introduced a
> per CPU variable xfd_state to keep the MSR_IA32_XFD value cached. In
> order to avoid unnecessary writes to the MSR.
>
> On CPU hotplug MSR_IA32_XFD is reset to the init_fpstate.xfd, which
> wipes out any stale state. But the per CPU cached xfd value is not
> reset, which brings them out of sync.
>
> As a consequence a subsequent xfd_update_state() might fail to update
> the MSR which in turn can result in XRSTOR raising a #NM in kernel
> space, which crashes the kernel.
>
> To address the issue mentioned, initialize xfd_state together with
> MSR_IA32_XFD.
>
> Fixes: 672365477ae8 ("x86/fpu: Update XFD state where required")
>
> Signed-off-by: Adamos Ttofari <attofari@xxxxxxxxx>

Tested-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>

With this test -- which I may follow up to be included the AMX selftest:
https://lore.kernel.org/lkml/6ab71997-8533-1828-7c62-717e2821f147@xxxxxxxxx/

Thanks,
Chang