From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sebastian Andrzej Siewior Subject: [PATCH v9 00/27] x86: load FPU registers on return to userland Date: Wed, 3 Apr 2019 18:41:29 +0200 Message-ID: <20190403164156.19645-1-bigeasy@linutronix.de> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Cc: x86@kernel.org, Andy Lutomirski , Paolo Bonzini , =?UTF-8?q?Radim=20Kr=C4=8Dm=C3=A1=C5=99?= , kvm@vger.kernel.org, "Jason A. Donenfeld" , Rik van Riel , Dave Hansen To: linux-kernel@vger.kernel.org Return-path: Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org This is a refurbished series originally started by by Rik van Riel. The goal is load the FPU registers on return to userland and not on every context switch. By this optimisation we can: - avoid loading the registers if the task stays in kernel and does not return to userland - make kernel_fpu_begin() cheaper: it only saves the registers on the first invocation. The second invocation does not need save them again. To access the FPU registers in kernel we need: - disable preemption to avoid that the scheduler switches tasks. By doing so it would set TIF_NEED_FPU_LOAD and the FPU registers would be not valid. - disable BH because the softirq might use kernel_fpu_begin() and then set TIF_NEED_FPU_LOAD instead loading the FPU registers on completion. v8…v9: Rebased on top of v5.1-rc3 plus: - using the proper register pointer in __fpu__restore_sig() for the fsave case. - dropped a WARN_ON() from switch_fpu_finish() if the PKRU state is missing. On a preemptible kernel this warning will trigger if the task gets preempted in fpstate_init() during memset() and fpstate_init_xstate(). - Adding a fastpath to __fpu__restore_sig() and copy_fpstate_to_sigframe() save/restore FPU state to/from userland. Thomas Gleixner asked for this. The series first does the slowpath and then the last few patches add/enable the fast path. This ensures that the slow works correctly (during a bisect) and then it should be rarely used. Some numbers generated by taskset 2 lat_sig -P 1 -W 64 -N 5000 catch v5.1-rc3 catch 64 Signal handler overhead: 1.7531 microseconds Signal handler overhead: 1.7547 microseconds Signal handler overhead: 1.7362 microseconds catch 32 Signal handler overhead: 2.4044 microseconds Signal handler overhead: 2.3732 microseconds Signal handler overhead: 2.3979 microseconds up to x86/fpu: Defer FPU state load until return to userspace catch 64 Signal handler overhead: 2.7556 microseconds Signal handler overhead: 2.7878 microseconds Signal handler overhead: 2.7664 microseconds catch 32 Signal handler overhead: 2.5961 microseconds Signal handler overhead: 2.5916 microseconds Signal handler overhead: 2.6091 microseconds up to x86/fpu: Add a fastpath to copy_fpstate_to_sigframe() catch 64 Signal handler overhead: 1.8573 microseconds Signal handler overhead: 1.8469 microseconds Signal handler overhead: 1.8554 microseconds catch 32 Signal handler overhead: 2.4875 microseconds Signal handler overhead: 2.4822 microseconds Signal handler overhead: 2.4806 microseconds ---- complete series catch 64 Signal handler overhead: 1.8463 microseconds Signal handler overhead: 1.8431 microseconds Signal handler overhead: 1.8609 microseconds catch 32 Signal handler overhead: 2.4907 microseconds Signal handler overhead: 2.4771 microseconds Signal handler overhead: 2.4939 microseconds v7…v8: Rebased on top of v5.1-rc1. And then: - Remove a WARN_ON() in switch_fpu_finish. Turns out it can trigger during a preemption while the xstate is initialized. - Provide __read_pkru_ins() and __write_pkru_ins which are symmetrical. __write_pkru() does the "write only if the value is different from current" check. - The kernel threads now load `init_pkru_value' instead of `0' for the PKRU value. - The PKRU value is also written into its xsave area of init_fpstate. v6…v7: Rebased on top of v5.0-rc7 and addressed Borislav Petkov's review. v5…v6: Rebased on top of v5.0-rc1. Integrated a few fixes which I noticed while looking over the patches, dropped the first few patches which were already applied. v4…v5: Rebased on top of a fix, noticed a problem with XSAVES and then redid the restore on sig return (patch #26 to #28). I don't like very much the sig save+restore thing that we are doing. It has been always like that. I *think* that this is just because we have nowhere to stash the FPU state while we are handling the signal. We could add another fpu->state for the signal handler and avoid the thing. Debian code-search revealed that `criu' is using it (and I didn't figure out why). Nothing else (that is packaged in Debian). Maybe we could get rid of this and if `criu' would then use a dedicated interface for its needs rather the signal interface that happen to do what it wants :) v3…v4: It has been suggested to remove the `initialized' member of the struct fpu because it should not required be needed with lazy-FPU-restore and would make the review easier. This is the first part of the series, the second is basically the rebase of the v3 queue. As a result, the diffstat became negative (which wasn't the case in previous version) :) I tried to incorporate all the review comments that came up, some of them were "outdated" after the removal of the `initialized' member. I'm sorry should I missed any. v1…v3: v2 was never posted. I followed the idea to completely decouple PKRU from xstate. This didn't quite work and made a few things complicated. One obvious required fixup is copy_fpstate_to_sigframe() where the PKRU state needs to be fiddled into xstate. This required another xfeatures_mask so that the sanity checks were performed and xstate_offsets would be computed. Additionally ptrace also reads/sets xstate in order to get/set the register and PKRU is one of them. So this would need some fiddle, too. In v3 I dropped that decouple idea. I also learned that the wrpkru instruction is not privileged and so caching it in kernel does not work. Instead I keep PKRU in xstate area and load it at context switch time while the remaining registers are deferred (until return to userland). The offset of PKRU within xstate is enumerated at boot time so why not use it. The following changes since commit 79a3aaa7b82e3106be97842dedfd8429248896e6: Linux 5.1-rc3 (2019-03-31 14:39:29 -0700) are available in the Git repository at: git://git.kernel.org/pub/scm/linux/kernel/git/bigeasy/staging.git x86_fpu_rtu_v9 for you to fetch changes up to c5265b05d167b3be19c787ebb913dc138485fa25: x86/pkeys: add PKRU value to init_fpstate (2019-04-03 18:19:10 +0200) ---------------------------------------------------------------- Rik van Riel (5): x86/fpu: Add (__)make_fpregs_active helpers x86/fpu: Eager switch PKRU state x86/fpu: Always store the registers in copy_fpstate_to_sigframe() x86/fpu: Prepare copy_fpstate_to_sigframe() for TIF_NEED_FPU_LOAD x86/fpu: Defer FPU state load until return to userspace Sebastian Andrzej Siewior (22): x86/fpu: Remove fpu->initialized usage in __fpu__restore_sig() x86/fpu: Remove fpu__restore() x86/fpu: Remove preempt_disable() in fpu__clear() x86/fpu: Always init the `state' in fpu__clear() x86/fpu: Remove fpu->initialized usage in copy_fpstate_to_sigframe() x86/fpu: Don't save fxregs for ia32 frames in copy_fpstate_to_sigframe() x86/fpu: Remove fpu->initialized x86/fpu: Remove user_fpu_begin() x86/fpu: Make __raw_xsave_addr() use feature number instead of mask x86/fpu: Make get_xsave_field_ptr() and get_xsave_addr() use feature number instead of mask x86/pkru: Provide .*_pkru_ins() functions x86/fpu: Only write PKRU if it is different from current x86/pkeys: Don't check if PKRU is zero before writting it x86/entry: Add TIF_NEED_FPU_LOAD x86/fpu: Update xstate's PKRU value on write_pkru() x86/fpu: Inline copy_user_to_fpregs_zeroing() x86/fpu: Let __fpu__restore_sig() restore the !32bit+fxsr frame from kernel memory x86/fpu: Merge the two code paths in __fpu__restore_sig() x86/fpu: Add a fastpath to __fpu__restore_sig() x86/fpu: Add a fastpath to copy_fpstate_to_sigframe() x86/fpu: Restore FPU register in copy_fpstate_to_sigframe() in order to use the fastpath x86/pkeys: add PKRU value to init_fpstate Documentation/preempt-locking.txt | 1 - arch/x86/entry/common.c | 8 ++ arch/x86/ia32/ia32_signal.c | 17 ++- arch/x86/include/asm/fpu/api.h | 31 ++++++ arch/x86/include/asm/fpu/internal.h | 130 ++++++++++++++++------ arch/x86/include/asm/fpu/signal.h | 2 +- arch/x86/include/asm/fpu/types.h | 9 -- arch/x86/include/asm/fpu/xstate.h | 5 +- arch/x86/include/asm/pgtable.h | 28 ++++- arch/x86/include/asm/special_insns.h | 18 +++- arch/x86/include/asm/thread_info.h | 2 + arch/x86/include/asm/trace/fpu.h | 8 +- arch/x86/kernel/cpu/common.c | 5 + arch/x86/kernel/fpu/core.c | 194 ++++++++++++++++----------------- arch/x86/kernel/fpu/init.c | 2 - arch/x86/kernel/fpu/regset.c | 24 ++--- arch/x86/kernel/fpu/signal.c | 202 +++++++++++++++++++++-------------- arch/x86/kernel/fpu/xstate.c | 42 ++++---- arch/x86/kernel/process.c | 2 +- arch/x86/kernel/process_32.c | 11 +- arch/x86/kernel/process_64.c | 11 +- arch/x86/kernel/signal.c | 17 ++- arch/x86/kernel/traps.c | 2 +- arch/x86/kvm/vmx/vmx.c | 2 +- arch/x86/kvm/x86.c | 48 +++++---- arch/x86/math-emu/fpu_entry.c | 3 - arch/x86/mm/mpx.c | 6 +- arch/x86/mm/pkeys.c | 21 ++-- 28 files changed, 496 insertions(+), 355 deletions(-) Sebastian