linux-kernel - Re: [PATCH v4 0/4] x86/speculation: Disable IBRS when idle

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Message-ID: <CAJZ5v0gL9eMY+rvRByp-ZSnhHXjbZWBd4a8Ny6ZLZ7OvmtRQ1Q@mail.gmail.com>
Date:   Wed, 28 Jun 2023 19:13:08 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Len Brown <lenb@...nel.org>, Jonathan Corbet <corbet@....net>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        x86@...nel.org, linux-pm@...r.kernel.org,
        Robin Jarry <rjarry@...hat.com>, Joe Mario <jmario@...hat.com>
Subject: Re: [PATCH v4 0/4] x86/speculation: Disable IBRS when idle

On Wed, Jun 28, 2023 at 4:27 AM Waiman Long <longman@...hat.com> wrote:
>
>  v4:
>   - Add a new __update_spec_ctrl() helper in patch 1.
>   - Rebased to the latest linux kernel.
>
>  v3:
>   - Drop patches 1 ("x86/speculation: Provide a debugfs file to dump
>     SPEC_CTRL MSRs") and 5 ("x86/idle: Disable IBRS entering mwait idle
>     and enable it on wakeup") for now.
>   - Drop the MSR restoration code in ("x86/idle: Disable IBRS when cpu
>     is offline") as native_play_dead() does not return.
>   - For patch ("intel_idle: Add ibrs_off module parameter to force
>     disable IBRS"), change the name from "no_ibrs" to "ibrs_off" and
>     document the new parameter in intel_idle.rst.
>
> For Intel processors that need to turn on IBRS to protect against
> Spectre v2 and Retbleed, the IBRS bit in the SPEC_CTRL MSR affects
> the performance of the whole core even if only one thread is turning
> it on when running in the kernel. For user space heavy applications,
> the performance impact of occasionally turning IBRS on during syscalls
> shouldn't be significant. Unfortunately, that is not the case when the
> sibling thread is idling in the kernel. In that case, the performance
> impact can be significant.
>
> When DPDK is running on an isolated CPU thread processing network packets
> in user space while its sibling thread is idle. The performance of the
> busy DPDK thread with IBRS on and off in the sibling idle thread are:
>
>                                 IBRS on         IBRS off
>                                 -------         --------
>   packets/second:                  7.8M           10.4M
>   avg tsc cycles/packet:         282.26          209.86
>
> This is a 25% performance degradation. The test system is a Intel Xeon
> 4114 CPU @ 2.20GHz.
>
> Commit bf5835bcdb96 ("intel_idle: Disable IBRS during long idle")
> disables IBRS when the CPU enters long idle (C6 or below). However, there
> are existing users out there who have set "intel_idle.max_cstate=1"
> to decrease latency. Those users won't be able to benefit from this
> commit. This patch series extends this commit by providing a new
> "intel_idle.ibrs_off" module parameter to force disable IBRS even when
> "intel_idle.max_cstate=1" at the expense of increased IRQ response
> latency. It also includes a commit to allow the disabling of IBRS when
> a CPU becomes offline.
>
>
> Waiman Long (4):
>   x86/speculation: Add __update_spec_ctrl() helper
>   x86/idle: Disable IBRS when cpu is offline
>   intel_idle: Use __update_spec_ctrl() in intel_idle_ibrs()
>   intel_idle: Add ibrs_off module parameter to force disable IBRS

x86 maintainers, if you want to take care of this series, please feel
free to add

Acked-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

to all of the patches in it.

Thanks!