Re: [PATCH v4 00/19] Enable FRED with KVM VMX

[Xin Li]

On 3/28/2025 10:11 AM, Xin Li (Intel) 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
>
> Following is the link to the v3 of this patch set:
> https://lore.kernel.org/lkml/20241001050110.3643764-1-xin@xxxxxxxxx/
>
> Since several preparatory patches in v3 have been merged, and Sean
> reiterated that it's NOT worth to precisely track which fields are/
> aren't supported [1], v4 patch number is reduced to 19.
>
> 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 [2].

Hi Sean,

Any chance we could merge FRED ahead of CET?

Ofc with proper changes to FRED code.

Thanks!
    Xin

> [1]: https://lore.kernel.org/lkml/Z73uK5IzVoBej3mi@xxxxxxxxxx/
> [2]: https://lore.kernel.org/kvm/ZvQaNRhrsSJTYji3@xxxxxxxxxx/