FRED, i.e., the Intel flexible return and event delivery architecture,
defines simple new transitions that change privilege level (ring
transitions).
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.
WRMSRNS is an instruction that behaves exactly like WRMSR, with the
only difference being that it is not a serializing instruction by
default. Under certain conditions, WRMSRNS may replace WRMSR to improve
performance. FRED uses it to switch RSP0 in a faster manner.
Search for the latest FRED spec in most search engines with this search
pattern:
site:intel.com FRED (flexible return and event delivery) specification
The CPUID feature flag CPUID.(EAX=7,ECX=1):EAX[17] enumerates FRED, and
the CPUID feature flag CPUID.(EAX=7,ECX=1):EAX[18] enumerates LKGS, and
the CPUID feature flag CPUID.(EAX=7,ECX=1):EAX[19] enumerates WRMSRNS.
Add CPUID definitions for FRED/LKGS/WRMSRNS, and expose them to KVM guests.
Because FRED relies on LKGS and WRMSRNS, add that to feature dependency
map.
Tested-by: Shan Kang <shan.kang@intel.com>
Signed-off-by: Xin Li <xin3.li@intel.com>
Message-ID: <20231109072012.8078-2-xin3.li@intel.com>
[Fix order of dependencies, add dependencies from LM to FRED. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
c1acad9f72