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Subject: Re: [PATCH v2 4/4] arch/x86: Add L1D flushing Documentation
To:     Balbir Singh <sblbir@amazon.com>, tglx@linutronix.de,
        linux-kernel@vger.kernel.org
Cc:     jpoimboe@redhat.com, tony.luck@intel.com, keescook@chromium.org,
        benh@kernel.crashing.org, x86@kernel.org, dave.hansen@intel.com
References: <20200406031946.11815-1-sblbir@amazon.com>
 <20200406031946.11815-5-sblbir@amazon.com>
From:   Randy Dunlap <rdunlap@infradead.org>
Message-ID: <5def424d-c7d5-c6fa-60b9-363f6bca6bc6@infradead.org>
Date:   Tue, 19 May 2020 08:39:49 -0700
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Hi--

Comments below. Sorry about the delay.

On 4/5/20 8:19 PM, Balbir Singh wrote:
> Add documentation of l1d flushing, explain the need for the
> feature and how it can be used.
> 
> Signed-off-by: Balbir Singh <sblbir@amazon.com>
> ---
>  Documentation/admin-guide/hw-vuln/index.rst   |  1 +
>  .../admin-guide/hw-vuln/l1d_flush.rst         | 40 +++++++++++++++++++
>  2 files changed, 41 insertions(+)
>  create mode 100644 Documentation/admin-guide/hw-vuln/l1d_flush.rst

> diff --git a/Documentation/admin-guide/hw-vuln/l1d_flush.rst b/Documentation/admin-guide/hw-vuln/l1d_flush.rst
> new file mode 100644
> index 000000000000..73ee9e491a74
> --- /dev/null
> +++ b/Documentation/admin-guide/hw-vuln/l1d_flush.rst
> @@ -0,0 +1,40 @@
> +L1D Flushing for the paranoid
> +=============================
> +
> +With an increasing number of vulnerabilities being reported around data
> +leaks from L1D, a new user space mechanism to flush the L1D cache on
> +context switch is added to the kernel. This should help address
> +CVE-2020-0550 and for paranoid applications, keep them safe from any
> +yet to be discovered vulnerabilities, related to leaks from the L1D
> +cache.
> +
> +Tasks can opt in to this mechanism by using an architecture specific
> +prctl (x86 only at the moment).
> +
> +Related CVES

           CVEs

> +------------
> +At the present moment, the following CVEs can be addressed by this
> +mechanism
> +
> +    =============       ========================     ==================
> +    CVE-2020-0550       Improper Data Forwarding     OS related aspects
> +    =============       ========================     ==================
> +
> +Usage Guidelines
> +----------------
> +Applications can call ``arch_prctl(2)`` with one of these two arguments

end above sentence with period or colon (colon might require the following
bullet items to be indented -- I'm not sure about that).

> +
> +1. ARCH_SET_L1D_FLUSH - flush the L1D cache on context switch (out)
> +2. ARCH_GET_L1D_FLUSH - get the current state of the L1D cache flush, returns 1
> +   if set and 0 if not set.
> +
> +**NOTE**: The feature is disabled by default, applications to need to specifically

                                        default; applications need to

> +opt into the feature to enable it.
> +
> +Mitigation
> +----------
> +When ARCH_SET_L1D_FLUSH is enabled for a task, on switching tasks (when
> +the address space changes), a flush of the L1D cache is performed for
> +the task when it leaves the CPU. If the underlying CPU supports L1D
> +flushing in hardware, the hardware mechanism is used, otherwise a software
> +fallback, similar to the mechanism used by L1TF is used.
> 

thanks.
-- 
~Randy