linux-kernel - Re: [PATCH v9 00/22] Enable FRED with KVM VMX

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Message-Id: <9EF391B5-71E8-4A9C-BD55-B78B5DEE5638@zytor.com>
Date: Thu, 6 Nov 2025 09:35:13 -0800
From: Xin Li <xin@...or.com>
To: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org
Cc: pbonzini@...hat.com, seanjc@...gle.com, corbet@....net, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, luto@...nel.org, peterz@...radead.org,
        andrew.cooper3@...rix.com, chao.gao@...el.com, hch@...radead.org,
        sohil.mehta@...el.com
Subject: Re: [PATCH v9 00/22] Enable FRED with KVM VMX


> On Oct 26, 2025, at 1:18 PM, Xin Li (Intel) <xin@...or.com> wrote:
> 
> This patch set enables the Intel flexible return and event delivery
> (FRED) architecture with KVM VMX to allow guests to utilize FRED.
> 
> 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.
> 
> 
> Intel VMX architecture is extended to run FRED guests, and the major
> changes are:
> 
> 1) New VMCS fields for FRED context management, which includes two new
> event data VMCS fields, eight new guest FRED context VMCS fields and
> eight new host FRED context VMCS fields.
> 
> 2) VMX nested-exception support for proper virtualization of stack
> levels introduced with FRED architecture.
> 
> Search for the latest FRED spec in most search engines with this search
> pattern:
> 
>  site:intel.com FRED (flexible return and event delivery) specification
> 
> 
> Although FRED and CET supervisor shadow stacks are independent CPU
> features, FRED unconditionally includes FRED shadow stack pointer
> MSRs IA32_FRED_SSP[0123], and IA32_FRED_SSP0 is just an alias of the
> CET MSR IA32_PL0_SSP.  IOW, the state management of MSR IA32_PL0_SSP
> becomes an overlap area, and Sean requested that FRED virtualization
> to land after CET virtualization [1].
> 
> With CET virtualization now merged in v6.18, the path is clear to submit
> the FRED virtualization patch series :).

Sean, what is the plan for the FRED patch series?

A good news is that we have got acks on all 3 common x86 patches.

Thanks!
Xin

> 
> Changes in v9:
> * Rebased to the latest kvm-x86/next branch, tag kvm-x86-next-2025.10.20-2.
> * Guard FRED state save/restore with guest_cpu_cap_has(vcpu, X86_FEATURE_FRED)
>  in patch 19 (syzbot & Chao).
> * Use array indexing for exception stack access, eliminating the need for
>  the ESTACKS_MEMBERS() macro in struct cea_exception_stacks, and then
>  exported __this_cpu_ist_top_va() in a subsequent patch (Dave Hansen).
> * Rewrote some of the change logs.