Re: [RFC PATCH 00/10] KVM: selftests: Add support for test-selectable ucall implementations

[Michael Roth <michael.roth@amd.com>]

On Wed, Jan 05, 2022 at 07:40:57PM +0000, Sean Christopherson wrote:
> On Wed, Jan 05, 2022, Michael Roth wrote:
> > On Wed, Jan 05, 2022 at 05:43:21PM +0000, Sean Christopherson wrote:
> > > Because it uses multiple VMs, and my rough sketch only allows for a single VM to
> > > use ucall.  Though I suppose we could simply keep appending to the ucall list for
> > > every VM.  The requirement would then be that all VMs are of the same type, i.e.
> > > utilize the same ucall_ops.
> > 
> > Hmm, maybe I misread your patch. Not supporting multiple VMs was the
> > reason I gave up on having the ucall structs allocated on-demand and
> > went with requiring them to be passed as arguments to ucall().
> > 
> > I thought with your patch you had solved that by having each vm have it's
> > own pool, via vm->ucall_list, and then mapping each pool into each guest
> > separately via:
> > 
> >   ucall_init(vm):
> >     ucall_list = vm->ucall_list
> >     sync_global_to_guest(ucall_list).
> > 
> > then as long as that ucall_init() is done *after* the guest calls
> > kvm_vm_elf_load(), it will end up with a 'ucall_list' global that points
> > to it's own specific vm->ucall_list. Then on the test side it doesn't
> > matter what the 'ucall_list' global is currently set to since you have
> > the GPA and know what vm exited.
> > 
> > Or am I missing something there?
> 
> Ha, that was not at all intented.  But yes, it should work.  I'd rather be lucky
> than good?

:)

> 
> > Although even if that is the case, now that we're proposing doing the
> > ucall_init() inside vm_create(), then we run the risk of a test calling
> > kvm_vm_elf_load() after, which might clobber the guest's copy of
> > ucall_list global if ucall_init() had since been called for another VM.
> > But that could maybe be worked around by having whatever vm_create()
> > variant we use also do the kvm_vm_elf_load() unconditionally as part of
> > creation.
> 
> Will sync_global_to_guest() even work as intended if kvm_vm_elf_load() hasn't
> been called?  If not, then sync_global_{to,from}_guest() should really assert if
> the test hasn't been loaded.

Yah, seems like it would get clobbered by kvm_vm_elf_load() later. And
can't think of any good reason to use sync_global_to_guest() without also
needing kvm_vm_elf_load() at some point, so makes sense to enforce it.

> 
> As for ucall_init(), I think the best approach would be to make kvm_vm_elf_load()
> a static and replace all calls with:
> 
> 	kvm_vm_load_guest(vm);
> 
> where its implementation is:
> 
>   void kvm_vm_load_guest(struct kvm_vm *vm)
>   {
>   	kvm_vm_elf_load(vm, program_invocation_name);
> 
> 	ucall_init(vm);
>   }
> 
> The logic being that if a test creates a VM but never loads any code into the guest,
> e.g. kvm_create_max_vcpus, then it _can't_ make ucalls.

Makes sense. And if different ops are needed for vmgexit()/tdcall() it
could be something like (if based on patches 1-5 of this series, and
extending vm_guest_mode as you suggested earlier):

   void kvm_vm_load_guest(struct kvm_vm *vm)
   {

     kvm_vm_elf_load(vm, program_invocation_name);
  
     if (vm->mode == VM_MODE_SEV)
  	    ucall_init_ops(vm, ucall_ops_pio_vmgexit);
     else (vm->vm_type == VM_MODE_TDX)
  	    ucall_init_ops(vm, ucall_ops_pio_tdcall);
     else
  	    ucall_init_ops(vm, ucall_ops_pio);

Shame we have to update all the kvm_vm_elf_load() call-sites, but
they'd end up potentially breaking things if left as-is anyway.

Were you planning on sending patches for these changes, or should I incorporate
your prototype and take a stab at the other changes as part of v2 of this
series?