Re: [PATCH 1/5] KVM: x86: Get vmcs12 pages before checking pending interrupts

[Mingwei Zhang]

On Thu, Aug 25, 2022 at 7:41 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Wed, Aug 24, 2022, Jim Mattson wrote:
> > On Wed, Aug 24, 2022 at 5:11 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> >
> > > @google folks, what would it take for us to mark KVM_REQ_GET_NESTED_STATE_PAGES
> > > as deprecated in upstream and stop accepting patches/fixes?  IIUC, when we eventually
> > > move to userfaultfd, all this goes away, i.e. we do want to ditch this at some point.
> >
> > Userfaultfd is a red herring. There were two reasons that we needed
> > this when nested live migration was implemented:
> > 1) our netlink socket mechanism for funneling remote page requests to
> > a userspace listener was broken.
> > 2) we were not necessarily prepared to deal with remote page requests
> > during VM setup.
> >
> > (1) has long since been fixed. Though our preference is to exit from
> > KVM_RUN and get the vCPU thread to request the remote page itself, we
> > are now capable of queuing a remote page request with a separate
> > listener thread and blocking in the kernel until the page is received.
> > I believe that mechanism is functionally equivalent to userfaultfd,
> > though not as elegant.
> > I don't know about (2). I'm not sure when the listener thread is set
> > up, relative to all of the other setup steps. Eliminating
> > KVM_REQ_GET_NESTED_STATE_PAGES means that userspace must be prepared
> > to fetch a remote page by the first call to KVM_SET_NESTED_STATE. The
> > same is true when using userfaultfd.
> >
> > These new ordering constraints represent a UAPI breakage, but we don't
> > seem to be as concerned about that as we once were. Maybe that's a
> > good thing. Can we get rid of all of the superseded ioctls, like
> > KVM_SET_CPUID, while we're at it?
>
> I view KVM_REQ_GET_NESTED_STATE_PAGES as a special case.  We are likely the only
> users, we can (eventually) wean ourselves off the feature, and we can carry
> internal patches (which we are obviously already carrying) until we transition
> away.  And unlike KVM_SET_CPUID and other ancient ioctls() that are largely
> forgotten, this feature is likely to be a maintenance burden as long as it exists.

KVM_REQ_GET_NESTED_STATE_PAGES has been uniformly used in
KVM_SET_NESTED_STATE ioctl in VMX (including eVMCS) and SVM, it is
basically a two-step setting up of a nested state mechanism.

We can change that, but this may have side effects and I think this
usage case has nothing to do with demand paging.

I noticed that nested_vmx_enter_non_root_mode() is called in
KVM_SET_NESTED_STATE in VMX, while in SVM implementation, it is simply
just a kvm_make_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu);

hmm... so is the nested_vmx_enter_non_root_mode() call in vmx
KVM_SET_NESTED_STATE ioctl() still necessary? I am thinking that
because the same function is called again in nested_vmx_run().

Thanks.
-Mingwei