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/
