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

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Message-ID: <d856b68d-3721-4d76-922b-4c98e2eb6c67@intel.com>
Date: Fri, 16 Jan 2026 11:49:53 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: "Xin Li (Intel)" <xin@...or.com>, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org, linux-doc@...r.kernel.org
Cc: pbonzini@...hat.com, seanjc@...gle.com, corbet@....net,
 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, luto@...nel.org,
 peterz@...radead.org, andrew.cooper3@...rix.com, chao.gao@...el.com,
 hch@...radead.org, sohil.mehta@...el.com
Subject: Re: [PATCH v9 08/22] KVM: VMX: Set FRED MSR intercepts

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.