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Date:   Wed, 20 Sep 2023 12:44:33 +0300
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Subject: Re: [PATCH v10 06/38] Documentation/x86/64: Add a documentation for
 FRED
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To:     Xin Li <xin3.li@intel.com>, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-edac@vger.kernel.org,
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References: <20230914044805.301390-1-xin3.li@intel.com>
 <20230914044805.301390-7-xin3.li@intel.com>
From:   Nikolay Borisov <nik.borisov@suse.com>
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On 14.09.23 г. 7:47 ч., Xin Li wrote:
> Briefly introduce FRED, and its advantages compared to IDT.
> 
> Signed-off-by: Xin Li <xin3.li@intel.com>
> ---
>   Documentation/arch/x86/x86_64/fred.rst  | 98 +++++++++++++++++++++++++
>   Documentation/arch/x86/x86_64/index.rst |  1 +
>   2 files changed, 99 insertions(+)
>   create mode 100644 Documentation/arch/x86/x86_64/fred.rst
> 
> diff --git a/Documentation/arch/x86/x86_64/fred.rst b/Documentation/arch/x86/x86_64/fred.rst
> new file mode 100644
> index 000000000000..a4ebb95f92c8
> --- /dev/null
> +++ b/Documentation/arch/x86/x86_64/fred.rst
> @@ -0,0 +1,98 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +=========================================
> +Flexible Return and Event Delivery (FRED)
> +=========================================
> +
> +Overview
> +========
> +
> +The FRED architecture defines simple new transitions that change
> +privilege level (ring transitions). The FRED architecture was
> +designed with the following goals:
> +
> +1) Improve overall performance and response time by replacing event
> +   delivery through the interrupt descriptor table (IDT event
> +   delivery) and event return by the IRET instruction with lower
> +   latency transitions.
> +
> +2) Improve software robustness by ensuring that event delivery
> +   establishes the full supervisor context and that event return
> +   establishes the full user context.
> +
> +The new transitions defined by the FRED architecture are FRED event
> +delivery and, for returning from events, two FRED return instructions.
> +FRED event delivery can effect a transition from ring 3 to ring 0, but
> +it is used also to deliver events incident to ring 0. One FRED
> +instruction (ERETU) effects a return from ring 0 to ring 3, while the
> +other (ERETS) returns while remaining in ring 0. Collectively, FRED
> +event delivery and the FRED return instructions are FRED transitions.
> +
> +In addition to these transitions, the FRED architecture defines a new
> +instruction (LKGS) for managing the state of the GS segment register.
> +The LKGS instruction can be used by 64-bit operating systems that do
> +not use the new FRED transitions.
> +
> +Furthermore, the FRED architecture is easy to extend for future CPU
> +architectures.
> +
> +Software based event dispatching
> +================================
> +
> +FRED operates differently from IDT in terms of event handling. Instead
> +of directly dispatching an event to its handler based on the event
> +vector, FRED requires the software to dispatch an event to its handler
> +based on both the event's type and vector. Therefore, an event dispatch
> +framework must be implemented to facilitate the event-to-handler
> +dispatch process. The FRED event dispatch framework takes control
> +once an event is delivered, and employs a two-level dispatch.
> +
> +The first level dispatching is event type based, and the second level
> +dispatching is event vector based.
> +
> +Full supervisor/user context
> +============================
> +
> +FRED event delivery atomically save and restore full supervisor/user
> +context upon event delivery and return. Thus it avoids the problem of
> +transient states due to %cr2 and/or %dr6, and it is no longer needed
> +to handle all the ugly corner cases caused by half baked entry states.
> +
> +FRED allows explicit unblock of NMI with new event return instructions
> +ERETS/ERETU, avoiding the mess caused by IRET which unconditionally
> +unblocks NMI, e.g., when an exception happens during NMI handling.
> +
> +FRED always restores the full value of %rsp, thus ESPFIX is no longer
> +needed when FRED is enabled.
> +
> +LKGS
> +====
> +
> +LKGS behaves like the MOV to GS instruction except that it loads the
> +base address into the IA32_KERNEL_GS_BASE MSR instead of the GS
> +segment’s descriptor cache. With LKGS, it ends up with avoiding
> +mucking with kernel GS, i.e., an operating system can always operate
> +with its own GS base address.
> +
> +Because FRED event delivery from ring 3 swaps the value of the GS base
> +address and that of the IA32_KERNEL_GS_BASE MSR, and ERETU swaps the
> +value of the GS base address and that of the IA32_KERNEL_GS_BASE MSR,
> +plus the introduction of LKGS instruction, the SWAPGS instruction is
> +no longer needed when FRED is enabled, thus is disallowed (#UD).

nit: This will be more clear if rewritten: "Because FRED event delivery 
from ring 3 and ERETU both swap the value of the GS base, plus the..." .

The idea is to remove the duplicate statement that IA32_KERNEL_GS_BASE 
and the GS registers are swapped as it makes the sentence somewhat hard 
to read.


<snip>