----- rkrcmar@xxxxxxxxxx wrote:
> 2018-03-12 13:12+0200, Liran Alon:
> > If KVM enable_vmware_backdoor module parameter is set,
> > the commit change VMX to now intercept #GP instead of being
> directly
> > deliviered from CPU to guest.
> >
> > It is done to support access to VMware backdoor I/O ports
> > even if TSS I/O permission denies it.
> > In that case:
> > 1. A #GP will be raised and intercepted.
> > 2. #GP intercept handler will simulate I/O port access instruction.
> > 3. I/O port access instruction simulation will allow access to
> VMware
> > backdoor ports specifically even if TSS I/O permission bitmap denies
> it.
> >
> > Note that the above change introduce slight performance hit as now
> #GPs
> > are not deliviered directly from CPU to guest but instead
> > cause #VMExit and instruction emulation.
> > However, this behavior is introduced only when
> enable_vmware_backdoor
> > KVM module parameter is set.
> >
> > Signed-off-by: Liran Alon <liran.alon@xxxxxxxxxx>
> > Reviewed-by: Nikita Leshenko <nikita.leshchenko@xxxxxxxxxx>
> > Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > Signed-off-by: Krish Sadhukhan <krish.sadhukhan@xxxxxxxxxx>
> > Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > ---
> > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > @@ -6141,6 +6154,17 @@ static int handle_exception(struct kvm_vcpu
> *vcpu)
> > if (intr_info & INTR_INFO_DELIVER_CODE_MASK)
> > error_code = vmcs_read32(VM_EXIT_INTR_ERROR_CODE);
> >
> > + if (!vmx->rmode.vm86_active && is_gp_fault(intr_info)) {
>
> Do we have some guarantees that the backdoor ports are never going to
> be
> used while vmx->rmode.vm86_active?
It's not because I can guarantee backdoor port not used in that mode.
It's because in that case, handle_exception() will call handle_rmode_exception()
which will call emulate_instruction() in case #GP(0) (Which should always be the
case when accessing VMware backdoor port).
>
> Thanks.
>
> > + WARN_ON_ONCE(!enable_vmware_backdoor);
> > + er = emulate_instruction(vcpu,
> > + EMULTYPE_VMWARE | EMULTYPE_NO_UD_ON_FAIL);
> > + if (er == EMULATE_USER_EXIT)
> > + return 0;
> > + else if (er != EMULATE_DONE)
> > + kvm_queue_exception_e(vcpu, GP_VECTOR, error_code);
> > + return 1;
> > + }
> > +
> > /*
> > * The #PF with PFEC.RSVD = 1 indicates the guest is accessing
> > * MMIO, it is better to report an internal error.
> > --
> > 1.9.1
> >
