linux-kernel - Re: [PATCH v2 0/3] KVM: EFER.LMSLE cleanup

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Message-ID: <YyrZOLq8z+lIORvP@zn.tnic>
Date:   Wed, 21 Sep 2022 11:28:24 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Jim Mattson <jmattson@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 0/3] KVM: EFER.LMSLE cleanup

On Tue, Sep 20, 2022 at 09:36:18PM +0000, Sean Christopherson wrote:
> Yes, but ideally KVM would explicitly tell the guest "you don't have LMSLE".
> Probably a moot point, but at the same time I don't see a reason not to be
> explicit.

Yes but...

On Tue, Sep 20, 2022 at 02:36:34PM -0700, Jim Mattson wrote:
> Reporting that CPUID bit gives us the right to raise #GP. AMD CPUs
> (going way back) that don't report EferLmsleUnsupported do not raise
> #GP.

... what does "gives us the right" mean exactly?

I'm pretty sure I'm missing something about how KVM works but wouldn't
it raise a guest #GP when the guest tries to set an unsupported EFER
bit? I.e., why do you need to explicitly do

	kvm_cpu_cap_set(X86_FEATURE_NO_LMSLE);

and not handle this like any other EFER reserved bit?

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette