Re: [PATCH v2 0/7] KVM: SVM: Enable FRED support

[Shivansh Dhiman]

Hi,

I'm planning to add another patch to this series in the next version. This is
related to dependency of FRED Virtualization on NMI Virtualization as mentioned
in the vFRED specification [1]. Any comments are welcome.

[1] https://docs.amd.com/v/u/en-US/69191-PUB

Thanks,
Shivansh

---

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 4b270490b2fd..f1fe407ea5da 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5379,6 +5379,9 @@ static __init void svm_set_cpu_caps(void)
        /* CPUID 0x8000001F (SME/SEV features) */
        sev_set_cpu_caps();

+       if (!vnmi)
+               kvm_cpu_cap_clear(X86_FEATURE_FRED);
+
        /*
         * Clear capabilities that are automatically configured by common code,
         * but that require explicit SVM support (that isn't yet implemented).



On 03-04-26 00:12, Shivansh Dhiman wrote:
> This series adds SVM support for FRED (Flexible Return and Event Delivery)
> virtualization in KVM.
> 
> FRED introduces simplified privilege level transitions to replace IDT-based
> event delivery and IRET returns, providing lower latency event handling while
> ensuring complete supervisor context on delivery and full user context on
> return. FRED defines event delivery for both ring 3->0 and ring 0->0
> transitions, and introduces ERETU for returning to ring 3 and ERETS for
> remaining in ring 0.
> 
> AMD hardware extends the VMCB to support FRED virtualization with dedicated
> save area fields for FRED MSRs (RSP0-3, SSP1-3, STKLVLS, CONFIG) and control
> fields for event injection data (EXITINTDATA, EVENTINJDATA).
> 
> The implementation spans seven patches. The important changes are:
> 
> 1) Extend VMCB structures with FRED fields mentioned above and disable MSR
>    interception for FRED-enabled guests to avoid unnecessary VM exits.
> 
> 2) Support for nested exceptions where we populate event injection data
>    when delivering exceptions like page faults and debug traps. 
> 
> Changes in v2:
> - Modified the zeroing of FRED MSRs from INIT to RESET and updated the commit
>   message (Sean Christopherson).
> - Used guest_cpu_cap_has() instead of checking VMCB bit in recalc intercepts 
>   (Sean Christopherson).
> - Variable rename from 'fred_enable' to 'intercept' (Sean Christopherson).
> - Moved FRED MSRs save/restore logic from svm_vcpu_enter_exit() to 
>   svm_prepare_[host_switch/switch_to_guest]() to reduce some MSR accesses.
> - While switching to host, added a safety check on guest_state_loaded.
> - Removed the need for a new function parameter 'reinject_on_vmexit' in 
>   svm_complete_interrupts() (Paolo Bonzini).
> - Replace variable 'nested' with 'is_nested' (Sean Christopherson).
> - Formatted variables in reverse fir-tree order (Sean Christopherson).
> - Gate all the FRED MSRs through guest_cpu_cap_has() while dumping VMCB
>   (Sean Christopherson).
> - Confined the enabling of vFRED to svm_vcpu_after_set_cpuid() only
>   (Sean Christopherson).
> 
> This series is based on top of FRED support for VMX patchset [1],
> patches 1-17. The VMX patchset was rebased on top of v6.18.0 kernel.
> Previous version v1 of this series can be found here:
> https://lore.kernel.org/kvm/20260129063653.3553076-1-shivansh.dhiman@xxxxxxx/
> 
> [1] https://lore.kernel.org/kvm/20251026201911.505204-1-xin@xxxxxxxxx
> 
> Regards,
> Shivansh
> ---
> Neeraj Upadhyay (3):
>   KVM: SVM: Initialize FRED VMCB fields
>   KVM: SVM: Populate FRED event data on event injection
>   KVM: SVM: Support FRED nested exception injection
> 
> Shivansh Dhiman (4):
>   KVM: SVM: Disable interception of FRED MSRs for FRED supported guests
>   KVM: SVM: Save restore FRED_RSP0 for FRED supported guests
>   KVM: SVM: Dump FRED context in dump_vmcb()
>   KVM: SVM: Enable FRED virtualization
> 
>  arch/x86/include/asm/svm.h |  35 ++++++++++-
>  arch/x86/kvm/svm/svm.c     | 122 ++++++++++++++++++++++++++++++++++++-
>  2 files changed, 152 insertions(+), 5 deletions(-)
> 
> base-commit: f76e83ecf6bce6d3793f828d92170b69e636f3c9