linux-kernel - Re: [PATCH 1/2] KVM: x86: Add emulation support for #GP triggered by VM instructions

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Date:   Thu, 14 Jan 2021 09:19:40 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Bandan Das <bsd@...hat.com>, Wei Huang <wei.huang2@....com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, vkuznets@...hat.com, joro@...tes.org,
        bp@...en8.de, tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
        jmattson@...gle.com, wanpengli@...cent.com, dgilbert@...hat.com
Subject: Re: [PATCH 1/2] KVM: x86: Add emulation support for #GP triggered by
 VM instructions

On Thu, Jan 14, 2021, Maxim Levitsky wrote:
> On Tue, 2021-01-12 at 15:00 -0500, Bandan Das wrote:
> > Sean Christopherson <seanjc@...gle.com> writes:
> > ...
> > > > -	if ((emulation_type & EMULTYPE_VMWARE_GP) &&
> > > > -	    !is_vmware_backdoor_opcode(ctxt)) {
> > > > -		kvm_queue_exception_e(vcpu, GP_VECTOR, 0);
> > > > -		return 1;
> > > > +	if (emulation_type & EMULTYPE_PARAVIRT_GP) {
> > > > +		vminstr = is_vm_instr_opcode(ctxt);
> > > > +		if (!vminstr && !is_vmware_backdoor_opcode(ctxt)) {
> > > > +			kvm_queue_exception_e(vcpu, GP_VECTOR, 0);
> > > > +			return 1;
> > > > +		}
> > > > +		if (vminstr)
> > > > +			return vminstr;
> > > 
> > > I'm pretty sure this doesn't correctly handle a VM-instr in L2 that hits a bad
> > > L0 GPA and that L1 wants to intercept.  The intercept bitmap isn't checked until
> > > x86_emulate_insn(), and the vm*_interception() helpers expect nested VM-Exits to
> > > be handled further up the stack.
> 
> Actually IMHO this exactly what we want. We want L0 to always intercept
> these #GPs, and hide them from the guest.
> 
> What we do need to do (and I prepared and attached a patch for that, is that
> if we run a guest, we want to inject corresponding vmexit (like
> SVM_EXIT_VMRUN) instead of emulating the instruction.

Yes, lack of forwarding to L1 as a nested exit is what I meant by "doesn't
correctly handle".