Linux Virtualization - Jun 2020 - [PATCH v3 59/75] x86/sev-es: Handle MONITOR/MONITORX Events

On Thu, Jun 11, 2020 at 03:10:45PM +0200, Joerg Roedel
wrote:
> On Tue, May 19, 2020 at 11:38:45PM -0700, Sean Christopherson wrote:
> > On Tue, Apr 28, 2020 at 05:17:09PM +0200, Joerg Roedel wrote:
> > > +static enum es_result vc_handle_monitor(struct ghcb *ghcb,
> > > +					struct es_em_ctxt *ctxt)
> > > +{
> > > +	phys_addr_t monitor_pa;
> > > +	pgd_t *pgd;
> > > +
> > > +	pgd = __va(read_cr3_pa());
> > > +	monitor_pa = vc_slow_virt_to_phys(ghcb, ctxt->regs->ax);
> > > +
> > > +	ghcb_set_rax(ghcb, monitor_pa);
> > > +	ghcb_set_rcx(ghcb, ctxt->regs->cx);
> > > +	ghcb_set_rdx(ghcb, ctxt->regs->dx);
> > > +
> > > +	return sev_es_ghcb_hv_call(ghcb, ctxt, SVM_EXIT_MONITOR, 0, 0);
> > 
> > Why?  If SVM has the same behavior as VMX, the MONITOR will be
disarmed on
> > VM-Enter, i.e. the VMM can't do anything useful for MONITOR/MWAIT.
I
> > assume that's the case given that KVM emulates MONITOR/MWAIT as
NOPs on
> > SVM.
> 
> Not sure if it is disarmed on VMRUN, but the MONITOR/MWAIT instructions
> are part of the GHCB spec, so they are implemented here.
Even if MONITOR/MWAIT somehow works across VMRUN I'm not sure it's
something
the guest should enable by default as it leaks GPAs to the untrusted host,
with no benefit to the guest except in specific configurations.  Yeah, the
VMM can muck with page tables to trace guest to the some extent, but the
guest shouldn't be unnecessarily volunteering information to the host.

If MONITOR/MWAIT is effectively a NOP then removing this code is a no
brainer.

Can someone from AMD chime in?

On 6/11/20 12:13 PM, Sean Christopherson wrote:
> On Thu, Jun 11, 2020 at 03:10:45PM +0200, Joerg Roedel wrote:
>> On Tue, May 19, 2020 at 11:38:45PM -0700, Sean Christopherson wrote:
>>> On Tue, Apr 28, 2020 at 05:17:09PM +0200, Joerg Roedel wrote:
>>>> +static enum es_result vc_handle_monitor(struct ghcb *ghcb,
>>>> +					struct es_em_ctxt *ctxt)
>>>> +{
>>>> +	phys_addr_t monitor_pa;
>>>> +	pgd_t *pgd;
>>>> +
>>>> +	pgd = __va(read_cr3_pa());
>>>> +	monitor_pa = vc_slow_virt_to_phys(ghcb,
ctxt->regs->ax);
>>>> +
>>>> +	ghcb_set_rax(ghcb, monitor_pa);
>>>> +	ghcb_set_rcx(ghcb, ctxt->regs->cx);
>>>> +	ghcb_set_rdx(ghcb, ctxt->regs->dx);
>>>> +
>>>> +	return sev_es_ghcb_hv_call(ghcb, ctxt, SVM_EXIT_MONITOR, 0,
0);
>>>
>>> Why?  If SVM has the same behavior as VMX, the MONITOR will be
disarmed on
>>> VM-Enter, i.e. the VMM can't do anything useful for
MONITOR/MWAIT.  I
>>> assume that's the case given that KVM emulates MONITOR/MWAIT as
NOPs on
>>> SVM.
>>
>> Not sure if it is disarmed on VMRUN, but the MONITOR/MWAIT instructions
>> are part of the GHCB spec, so they are implemented here.
> 
> Even if MONITOR/MWAIT somehow works across VMRUN I'm not sure it's
something
> the guest should enable by default as it leaks GPAs to the untrusted host,
> with no benefit to the guest except in specific configurations.  Yeah, the
> VMM can muck with page tables to trace guest to the some extent, but the
> guest shouldn't be unnecessarily volunteering information to the host.
> 
> If MONITOR/MWAIT is effectively a NOP then removing this code is a no
> brainer.
> 
> Can someone from AMD chime in?
I don't think there is any guarantee that MONITOR/MWAIT would work within 
a guest (I'd have to dig some more on that to get a definitive answer, but 
probably not necessary to do). As you say, if KVM emulates it as a NOP, 
there's no sense in exposing the GPA - make it a NOP in the handler. I 
just need to poke some other hypervisor vendors and hear what they have to 
say.

Thanks,
Tom

>

On Thu, Jun 11, 2020 at 02:33:12PM -0500, Tom Lendacky
wrote:
> I don't think there is any guarantee that MONITOR/MWAIT would work
within a
> guest (I'd have to dig some more on that to get a definitive answer,
but
> probably not necessary to do). As you say, if KVM emulates it as a NOP,
> there's no sense in exposing the GPA - make it a NOP in the handler. I
just
> need to poke some other hypervisor vendors and hear what they have to say.
Okay, makes sense. I made monitor/mwait nops in the patch-set.

Regards,

	Joerg