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Date: Tue, 22 Jun 2021 17:38:24 +0200
From: Borislav Petkov <bp@...e.de>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,
Andy Lutomirski <luto@...nel.org>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Fenghua Yu <fenghua.yu@...el.com>,
Tony Luck <tony.luck@...el.com>,
Yu-cheng Yu <yu-cheng.yu@...el.com>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>,
Kan Liang <kan.liang@...ux.intel.com>
Subject: Re: [patch V3 47/66] x86/fpu: Clean up the fpu__clear() variants
Only nitpicks:
On Fri, Jun 18, 2021 at 04:19:10PM +0200, Thomas Gleixner wrote:
> From: Andy Lutomirski <luto@...nel.org>
>
> 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@...nel.org>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
> 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= ¤t->thread.fpu;
ERROR: spaces required around that '=' (ctx:VxW)
#167: FILE: arch/x86/kernel/fpu/core.c:335:
+ struct fpu *fpu= ¤t->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 != ¤t->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.
SUSE Software Solutions Germany GmbH, GF: Felix Imendörffer, HRB 36809, AG Nürnberg
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