linux-kernel - RE: [PATCH v4 34/34] KVM: x86/vmx: execute "int $2" for NMI reinjection when FRED is enabled

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Date:   Mon, 6 Mar 2023 18:56:22 +0000
From:   "Li, Xin3" <xin3.li@...el.com>
To:     "Christopherson,, Sean" <seanjc@...gle.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>
Subject: RE: [PATCH v4 34/34] KVM: x86/vmx: execute "int $2" for NMI
 reinjection when FRED is enabled

> > Execute "int $2" for NMI reinjection when FRED is enabled.
> 
> This needs an explanation of _why_.  And as requested earlier[*], please avoid
> "reinjection" in this context.

W/ FRED, we use ERETS to unblock NMI when returning from a NMI handler
(Setting bit 28 of CS).  And there are 2 approaches to do it:
1) "int $2", let the h/w do the job for us.
2) create a FRED stack frame, and then jump to fred_entrypoint_kernel
in arch/x86/entry/entry_64_fred.S.

I prefer "INT $2", what is your opinion?

> [*] https://lore.kernel.org/all/Y20f8v9ObO+IPwU+@google.com