Re: [PATCH v9 08/22] KVM: VMX: Set FRED MSR intercepts

[Dave Hansen]

On 10/26/25 13:18, Xin Li (Intel) wrote:
> Both MSR_IA32_FRED_RSP0 and MSR_IA32_FRED_SSP0 (aka MSR_IA32_PL0_SSP)
> are dedicated for userspace event delivery, IOW they are NOT used in
> any kernel event delivery and the execution of ERETS.  Thus KVM can
> run safely with guest values in the two MSRs.  As a result, save and
> restore of their guest values are deferred until vCPU context switch,
> Host MSR_IA32_FRED_RSP0 is restored upon returning to userspace, and
> Host MSR_IA32_PL0_SSP is managed with XRSTORS/XSAVES.

Is it worth making MSR_IA32_FRED_RSP0 special versus MSR_IA32_FRED_RSP[123]?

Is it needed because MSR_IA32_FRED_RSP0 is rewritten all the time as
CPUs switch between threads? But MSR_IA32_FRED_RSP[123] are not
frequently written?

I'd like to hear more about the motivation.