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

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