On 10/26/2026 9:04 AM, Sean Christopherson wrote:
> And we'd want to keep the optimizations to elide writes when vCPUs are using
> the
> same value(s). So, compile tested only...
>
Will test this out on real hardware with the MSR_STAR change below.
>
> +static bool kvm_is_msr_used_in_host(u32 msr)
> +{
> + if (!cpu_feature_enabled(X86_FEATURE_FRED))
> + return true;
> +
> + switch (msr) {
> + case MSR_STAR:
AFAIU, MSR_STAR is still used with FRED so it shouldn't be included
here. The Spec says in section 4.4:
IA32_STAR:
- Existing use: this MSR is used by the existing operation of SYSCALL
and SYSRET.
- FRED use: FRED event delivery loads the CS and SS selectors with
values derived from IA32_STAR[47:32] (see Section 5.3). ERETU uses the
value of IA32_STAR[63:48] to determine how to load the CS and SS
registers (see Section 6.2.1).
It is expected that operating systems will set IA32_STAR[33:32] to 00B
and IA32_STAR[49:48] to 11B.
Likewise, the kernel also writes to MSR_STAR during syscall_init() and
it has a comment saying: "Except the IA32_STAR MSR, there is NO need to
setup SYSCALL and SYSENTER MSRs for FRED, .."
> + case MSR_LSTAR:
> + case MSR_CSTAR:
> + case MSR_SYSCALL_MASK:
> + return false;
> + default:
> + break;
> + }
> + return true;
> +}
> +
