Re: [PATCH v4 14/22] x86/fpu/xstate: Expand the xstate buffer on the first use of dynamic user state

[Jann Horn <jannh@google.com>]

On Sun, Feb 21, 2021 at 7:56 PM Chang S. Bae <chang.seok.bae@intel.com> wrote:
> Intel's Extended Feature Disable (XFD) feature is an extension of the XSAVE
> architecture. XFD allows the kernel to enable a feature state in XCR0 and
> to receive a #NM trap when a task uses instructions accessing that state.
> In this way, Linux can defer allocating the large XSAVE buffer until tasks
> need it.
>
> XFD introduces two MSRs: IA32_XFD to enable/disable the feature and
> IA32_XFD_ERR to assist the #NM trap handler. Both use the same
> state-component bitmap format, used by XCR0.
>
> Use this hardware capability to find the right time to expand the xstate
> buffer. Introduce two sets of helper functions for that:
>
> 1. The first set is primarily for interacting with the XFD hardware:
>         xdisable_setbits()
>         xdisable_getbits()
>         xdisable_switch()
>
> 2. The second set is for managing the first-use status and handling #NM
>    trap:
>         xfirstuse_enabled()
>         xfirstuse_not_detected()
>
> The #NM handler induces the xstate buffer expansion to save the first-used
> states.
[...]
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 7f5aec758f0e..821a7f408ad4 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
[...]
> +static __always_inline bool handle_xfirstuse_event(struct fpu *fpu)
> +{
> +       bool handled = false;
> +       u64 event_mask;
[...]
> +       if (alloc_xstate_buffer(fpu, event_mask))
> +               return handled;
[...]
> +}
> +
>  DEFINE_IDTENTRY(exc_device_not_available)
>  {
>         unsigned long cr0 = read_cr0();
>
> +       if (handle_xfirstuse_event(&current->thread.fpu))
> +               return;

What happens if handle_xfirstuse_event() fails because vmalloc()
failed in alloc_xstate_buffer()? I think that should probably kill the
task with something like force_sig() - but as far as I can tell, at
the moment, it will instead end up at die(), which should only be used
for kernel bugs.

> +
>  #ifdef CONFIG_MATH_EMULATION
>         if (!boot_cpu_has(X86_FEATURE_FPU) && (cr0 & X86_CR0_EM)) {
>                 struct math_emu_info info = { };
> --
> 2.17.1
>
>