From: Dave Hansen <dave.hansen@intel.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Valentin Schneider <vschneid@redhat.com>,
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Subject: Re: [PATCH v5 00/25] context_tracking,x86: Defer some IPIs until a user->kernel transition
Date: Wed, 30 Apr 2025 13:00:10 -0700 [thread overview]
Message-ID: <a6b3a331-1ff3-4490-b300-a62b3c21578d@intel.com> (raw)
In-Reply-To: <20250430154228.1d6306b4@gandalf.local.home>
On 4/30/25 12:42, Steven Rostedt wrote:
>> Look at the syscall code for instance:
>>
>>> SYM_CODE_START(entry_SYSCALL_64)
>>> swapgs
>>> movq %rsp, PER_CPU_VAR(cpu_tss_rw + TSS_sp2)
>>> SWITCH_TO_KERNEL_CR3 scratch_reg=%rsp
>> You can _trivially_ audit this and know that swapgs doesn't touch memory
>> and that as long as PER_CPU_VAR()s and the process stack don't have
>> their mappings munged and flushes deferred that this would be correct.
> Hmm, so there is still a path for this?
>
> At least if it added more ways to debug it, and some other changes to make
> the locations where vmalloc is dangerous smaller?
Being able to debug it would be a good start. But, more generally, what
we need is for more people to be able to run the code in the first
place. Would a _normal_ system (without setups that are trying to do
NOHZ_FULL) ever be able to defer TLB flush IPIs?
If the answer is no, then, yeah, I'll settle for some debugging options.
But if you shrink the window as small as I'm talking about, it would
look very different from this series.
For instance, imagine when a CPU goes into the NOHZ mode. Could it just
unconditionally flush the TLB on the way back into the kernel (in the
same SWITCH_TO_KERNEL_CR3 spot)? Yeah, it'll make entry into the kernel
expensive for NOHZ tasks, but it's not *THAT* bad. And if the entire
point of a NOHZ_FULL task is to minimize the number of kernel entries
then a little extra overhead there doesn't sound too bad.
Also, about the new hardware, I suspect there's some mystery customer
lurking in the shadows asking folks for this functionality. Could you at
least go _talk_ to the mystery customer(s) and see which hardware they
care about? They might already even have the magic CPUs they need for
this, or have them on the roadmap. If they've got Intel CPUs, I'd be
happy to help figure it out.
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next prev parent reply other threads:[~2025-04-30 20:00 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-04-29 11:32 [PATCH v5 00/25] context_tracking,x86: Defer some IPIs until a user->kernel transition Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 01/25] objtool: Make validate_call() recognize indirect calls to pv_ops[] Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 02/25] objtool: Flesh out warning related to pv_ops[] calls Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 03/25] rcu: Add a small-width RCU watching counter debug option Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 04/25] rcutorture: Make TREE04 use CONFIG_RCU_DYNTICKS_TORTURE Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 05/25] jump_label: Add annotations for validating noinstr usage Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 06/25] static_call: Add read-only-after-init static calls Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 07/25] x86/paravirt: Mark pv_sched_clock static call as __ro_after_init Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 08/25] x86/idle: Mark x86_idle " Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 09/25] x86/paravirt: Mark pv_steal_clock " Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 10/25] riscv/paravirt: " Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 11/25] loongarch/paravirt: " Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 12/25] arm64/paravirt: " Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 13/25] arm/paravirt: " Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 14/25] perf/x86/amd: Mark perf_lopwr_cb " Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 15/25] sched/clock: Mark sched_clock_running key " Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 16/25] KVM: VMX: Mark __kvm_is_using_evmcs static " Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 17/25] x86/speculation/mds: Mark mds_idle_clear key as allowed in .noinstr Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 18/25] sched/clock, x86: Mark __sched_clock_stable " Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 19/25] KVM: VMX: Mark vmx_l1d_should flush and vmx_l1d_flush_cond keys " Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 20/25] stackleack: Mark stack_erasing_bypass key " Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 21/25] objtool: Add noinstr validation for static branches/calls Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 22/25] module: Remove outdated comment about text_size Valentin Schneider
2025-05-05 14:27 ` Petr Pavlu
2025-04-29 11:32 ` [PATCH v5 23/25] module: Add MOD_NOINSTR_TEXT mem_type Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 24/25] context-tracking: Introduce work deferral infrastructure Valentin Schneider
2025-04-29 11:32 ` [PATCH v5 25/25] context_tracking,x86: Defer kernel text patching IPIs Valentin Schneider
2025-04-29 16:11 ` [PATCH v5 00/25] context_tracking,x86: Defer some IPIs until a user->kernel transition Dave Hansen
2025-04-30 17:20 ` Steven Rostedt
2025-04-30 18:07 ` Dave Hansen
2025-04-30 19:42 ` Steven Rostedt
2025-04-30 20:00 ` Dave Hansen [this message]
2025-05-02 9:55 ` Valentin Schneider
2025-05-02 13:53 ` Dave Hansen
2025-05-02 16:38 ` Valentin Schneider
2025-05-02 17:57 ` Dave Hansen
2025-05-05 15:45 ` Valentin Schneider
2025-05-02 11:22 ` Peter Zijlstra
2025-05-02 14:33 ` Dave Hansen
2025-05-02 15:20 ` Peter Zijlstra
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