Re: [PATCH v1 3/5] KVM: x86: nVMX: VMCS12 field's read/write respects field existence bitmap

[Yu Zhang <yu.c.zhang@linux.intel.com>]

On Tue, Oct 05, 2021 at 11:22:15PM +0000, Sean Christopherson wrote:
> On Tue, Oct 05, 2021, Jim Mattson wrote:
> > On Tue, Oct 5, 2021 at 1:50 PM Sean Christopherson <seanjc@google.com> wrote:
> > >
> > > On Tue, Oct 05, 2021, Jim Mattson wrote:
> > > > On Tue, Oct 5, 2021 at 10:59 AM Sean Christopherson <seanjc@google.com> wrote:
> > > > >
> > > > > On Tue, Oct 05, 2021, Jim Mattson wrote:
> > > > > > On Tue, Oct 5, 2021 at 9:16 AM Sean Christopherson <seanjc@google.com> wrote:
> > > > > > >
> > > > > > > On Tue, Sep 28, 2021, Robert Hoo wrote:
> > > > > > > > On Fri, 2021-09-03 at 15:11 +0000, Sean Christopherson wrote:
> > > > > > > >       You also said, "This is quite the complicated mess for
> > > > > > > > something I'm guessing no one actually cares about.  At what point do
> > > > > > > > we chalk this up as a virtualization hole and sweep it under the rug?"
> > > > > > > > -- I couldn't agree more.
> > > > > > >
> > > > > > > ...
> > > > > > >
> > > > > > > > So, Sean, can you help converge our discussion and settle next step?
> > > > > > >
> > > > > > > Any objection to simply keeping KVM's current behavior, i.e. sweeping this under
> > > > > > > the proverbial rug?
> > > > > >
> > > > > > Adding 8 KiB per vCPU seems like no big deal to me, but, on the other
> > > > > > hand, Paolo recently argued that slightly less than 1 KiB per vCPU was
> > > > > > unreasonable for VM-exit statistics, so maybe I've got a warped
> > > > > > perspective. I'm all for pedantic adherence to the specification, but
> > > > > > I have to admit that no actual hypervisor is likely to care (or ever
> > > > > > will).
> > > > >
> > > > > It's not just the memory, it's also the complexity, e.g. to get VMCS shadowing
> > > > > working correctly, both now and in the future.
> > > >
> > > > As far as CPU feature virtualization goes, this one doesn't seem that
> > > > complex to me. It's not anywhere near as complex as virtualizing MTF,
> > > > for instance, and KVM *claims* to do that! :-)
> > >
> > > There aren't many things as complex as MTF.  But unlike MTF, this behavior doesn't
> > > have a concrete use case to justify the risk vs. reward.  IMO the odds of us breaking
> > > something in KVM for "normal" use cases are higher than the odds of an L1 VMM breaking
> > > because a VMREAD/VMWRITE didn't fail when it technically should have failed.
> > 
> > Playing devil's advocate here, because I totally agree with you...
> > 
> > Who's to say what's "normal"? It's a slippery slope when we start
> > making personal value judgments about which parts of the architectural
> > specification are important and which aren't.
> 
> I agree, but in a very similar case Intel chose to take an erratum instead of
> fixing what was in all likelihood a microcode bug, i.e. could have been patched
> in the field.  So it's not _just_ personal value judgment, though it's definitely
> that too :-)
> 
> I'm not saying I'd actively oppose support for strict VMREAD/VMWRITE adherence
> to the vCPU model, but I'm also not going to advise anyone to go spend their time
> implementing a non-trivial fix for behavior that, AFAIK, doesn't adversely affect
> any real world use cases.
> 

Thank you all for the discussion, Sean & Jim.

Could we draw a conclusion to just keep KVM as it is now? If yes, how about we
depricate the check against max index value from MSR_IA32_VMX_VMCS_ENUM in vmx.c 
of the kvm-unit-test?

After all, we have not witnessed any real system doing so.

E.g.,

diff --git a/x86/vmx.c b/x86/vmx.c
index f0b853a..63623e5 100644
--- a/x86/vmx.c
+++ b/x86/vmx.c
@@ -380,8 +380,7 @@ static void test_vmwrite_vmread(void)
        vmcs_enum_max = (rdmsr(MSR_IA32_VMX_VMCS_ENUM) & VMCS_FIELD_INDEX_MASK)
                        >> VMCS_FIELD_INDEX_SHIFT;
        max_index = find_vmcs_max_index();
-       report(vmcs_enum_max == max_index,
-              "VMX_VMCS_ENUM.MAX_INDEX expected: %x, actual: %x",
+       printf("VMX_VMCS_ENUM.MAX_INDEX expected: %x, actual: %x",
               max_index, vmcs_enum_max);

        assert(!vmcs_clear(vmcs));

B.R.
Yu