Re: [patch V3 47/66] x86/fpu: Clean up the fpu__clear() variants

[Borislav Petkov]

Only nitpicks:

On Fri, Jun 18, 2021 at 04:19:10PM +0200, Thomas Gleixner wrote:
> From: Andy Lutomirski <luto@xxxxxxxxxx>
> 
> fpu__clear() currently resets both register state and kernel XSAVE buffer
> state.  It has two modes: one for all state (supervisor and user) and
> another for user state only.  fpu__clear_all() uses the "all state"
> (user_only=0) mode, while a number of signal paths use the user_only=1
> mode.
> 
> Make fpu__clear() work only for user state (user_only=1) and remove the
> "all state" (user_only=0) code.  Rename it to match so it can be used by
> the signal paths.
> 
> Replace the "all state" (user_only=0) fpu__clear() functionality.  Use the
> TIF_NEED_FPU_LOAD functionality instead of making any actual hardware
> registers changes in this path.
> 
> Instead of invoking fpu__initialize() just memcpy() init_fpstate into the
> tasks FPU state because that has already the correct format and in case of

task's

> PKRU also contains the default PKRU value. Move the actual PKRU write out
> into flush_thread() where it belongs and where it will end up anyway when
> PKRU and XSTATE have been distangled.

untangled

> 
> For bisectability a workaround is required which stores the PKRU value in
> the xstate memory until PKRU is distangled from XSTATE for context

untangled

> switching and return to user.
> 
> [ Dave Hansen: Polished changelog ]
> [ tglx: Fixed the PKRU fallout ]
> 
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
>  arch/x86/kernel/fpu/core.c |  111 ++++++++++++++++++++++++++++++---------------
>  arch/x86/kernel/process.c  |   10 ++++
>  2 files changed, 85 insertions(+), 36 deletions(-)

...

> +/* Temporary workaround. Will be removed once PKRU and XSTATE are distangled. */

untangled

> +static inline void pkru_set_default_in_xstate(struct xregs_state *xsave)
> +{
> +	struct pkru_state *pk;
> +
> +	if (!cpu_feature_enabled(X86_FEATURE_OSPKE))
> +		return;
> +	/*
> +	 * Force XFEATURE_PKRU to be set in the header otherwise
> +	 * get_xsave_addr() does not work and it also needs to be set to
> +	 * make XRSTOR(S) load it.
> +	 */
> +	xsave->header.xfeatures |= XFEATURE_MASK_PKRU;
> +	pk = get_xsave_addr(xsave, XFEATURE_PKRU);
> +	pk->pkru = pkru_get_init_value();
> +}
> +
>  /*
> - * Clear the FPU state back to init state.
> - *
> - * Called by sys_execve(), by the signal handler code and by various
> - * error paths.
> + * Reset current->fpu memory state to the init values.
>   */
> -static void fpu__clear(struct fpu *fpu, bool user_only)
> +static void fpu_reset_fpstate(void)
> +{
> +	struct fpu *fpu= &current->thread.fpu;

ERROR: spaces required around that '=' (ctx:VxW)
#167: FILE: arch/x86/kernel/fpu/core.c:335:
+	struct fpu *fpu= &current->thread.fpu;
 	               ^


> +
> +	fpregs_lock();
> +	fpu__drop(fpu);
> +	/*
> +	 * This does not change the actual hardware registers. It just
> +	 * resets the memory image and sets TIF_NEED_FPU_LOAD so a
> +	 * subsequent return to usermode will reload the registers from the
> +	 * tasks memory image.

task's

> +	 *
> +	 * Do not use fpstate_init() here. Just copy init_fpstate which has
> +	 * the correct content already except for PKRU.
> +	 */

Dunno, this comment can be over the function name - it is small enough
anyway.

> +	memcpy(&fpu->state, &init_fpstate, init_fpstate_copy_size());
> +	pkru_set_default_in_xstate(&fpu->state.xsave);
> +	set_thread_flag(TIF_NEED_FPU_LOAD);
> +	fpregs_unlock();
> +}
> +
> +/*
> + * Reset current's user FPU states to the init states.  current's
> + * supervisor states, if any, are not modified by this function.  The
> + * caller guarantees that the XSTATE header in memory is intact.
> + */
> +void fpu__clear_user_states(struct fpu *fpu)
>  {
>  	WARN_ON_FPU(fpu != &current->thread.fpu);
>  
> +	fpregs_lock();
>  	if (!static_cpu_has(X86_FEATURE_FPU)) {

cpu_feature_enabled()

> -		fpu__drop(fpu);
> -		fpu__initialize(fpu);
> +		fpu_reset_fpstate();
> +		fpregs_unlock();
>  		return;
>  	}
>  
> -	fpregs_lock();
> -
> -	if (user_only) {
> -		if (!fpregs_state_valid(fpu, smp_processor_id()) &&
> -		    xfeatures_mask_supervisor())
> -			os_xrstor(&fpu->state.xsave, xfeatures_mask_supervisor());
> -		load_fpregs_from_init_fpstate(xfeatures_mask_user());
> -	} else {
> -		load_fpregs_from_init_fpstate(xfeatures_mask_all);
> +	/*
> +	 * Ensure that current's supervisor states are loaded into their
> +	 * corresponding registers.
> +	 */
> +	if (xfeatures_mask_supervisor() &&
> +	    !fpregs_state_valid(fpu, smp_processor_id())) {
> +		os_xrstor(&fpu->state.xsave, xfeatures_mask_supervisor());
>  	}
>  
> +	/* Reset user states in registers. */
> +	load_fpregs_from_init_fpstate(xfeatures_mask_user());
> +
> +	/*
> +	 * Now all FPU registers have their desired values.  Inform the FPU
> +	 * state machine that current's FPU registers are in the hardware
> +	 * registers. The memory image does not need to be updated because
> +	 * any operation relying on it has to save the registers first when
> +	 * currents FPU is marked active.

current's

> +	 */
>  	fpregs_mark_activate();
>  	fpregs_unlock();
>  }
>  

-- 
Regards/Gruss,
    Boris.

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