linux-kernel - Re: [PATCH v2 02/11] KVM: x86: emulator: introduce update_emulation

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Message-ID: <YtiTeZQ/n0LPTV/W@google.com>
Date:   Wed, 20 Jul 2022 23:44:57 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org,
        Kees Cook <keescook@...omium.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
        Borislav Petkov <bp@...en8.de>, Joerg Roedel <joro@...tes.org>,
        Ingo Molnar <mingo@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH v2 02/11] KVM: x86: emulator: introduce
 update_emulation_mode

On Tue, Jun 21, 2022, Maxim Levitsky wrote:
> +static inline int update_emulation_mode(struct x86_emulate_ctxt *ctxt)

Maybe emulator_recalc_and_set_mode()?  It took me a second to understand that
"update" also involves determining the "new" mode, e.g. I was trying to figure
out where @mode was :-)

> +{
> +	u64 efer;
> +	struct desc_struct cs;
> +	u16 selector;
> +	u32 base3;
> +
> +	ctxt->ops->get_msr(ctxt, MSR_EFER, &efer);
> +
> +	if (!ctxt->ops->get_cr(ctxt, 0) & X86_CR0_PE) {
> +		/* Real mode. cpu must not have long mode active */
> +		if (efer & EFER_LMA)
> +			return X86EMUL_UNHANDLEABLE;

If we hit this, is there any hope of X86EMUL_UNHANDLEABLE doing the right thing?
Ah, SMM and the ability to swizzle SMRAM state.  Bummer.  I was hoping we could
just bug the VM.

> +		ctxt->mode = X86EMUL_MODE_REAL;
> +		return X86EMUL_CONTINUE;
> +	}