Re: [PATCH 2/4] KVM: selftests: Setup ucall after loading program into guest memory

[Sean Christopherson <seanjc@google.com>]

On Thu, Dec 08, 2022, Oliver Upton wrote:
> On Wed, Dec 07, 2022 at 11:57:27PM +0000, Sean Christopherson wrote:
> > > diff --git a/tools/testing/selftests/kvm/aarch64/page_fault_test.c b/tools/testing/selftests/kvm/aarch64/page_fault_test.c
> > > index 92d3a91153b6..95d22cfb7b41 100644
> > > --- a/tools/testing/selftests/kvm/aarch64/page_fault_test.c
> > > +++ b/tools/testing/selftests/kvm/aarch64/page_fault_test.c
> > > @@ -609,8 +609,13 @@ static void setup_memslots(struct kvm_vm *vm, struct test_params *p)
> > >  				    data_size / guest_page_size,
> > >  				    p->test_desc->data_memslot_flags);
> > >  	vm->memslots[MEM_REGION_TEST_DATA] = TEST_DATA_MEMSLOT;
> > > +}
> > > +
> > > +static void setup_ucall(struct kvm_vm *vm)
> > > +{
> > > +	struct userspace_mem_region *region = vm_get_mem_region(vm, MEM_REGION_TEST_DATA);
> > >  
> > > -	ucall_init(vm, data_gpa + data_size);
> > > +	ucall_init(vm, region->region.guest_phys_addr + region->region.memory_size);
> > 
> > Isn't there a hole after CODE_AND_DATA_MEMSLOT?  I.e. after memslot 0?
> 
> Sure, but that's only guaranteed in the PA space.
> 
> > The reason
> > I ask is because if so, then we can do the temporarily heinous, but hopefully forward
> > looking thing of adding a helper to wrap kvm_vm_elf_load() + ucall_init().
> > 
> > E.g. I think we can do this immediately, and then at some point in the 6.2 cycle
> > add a dedicated region+memslot for the ucall MMIO page.
> 
> Even still, that's just a kludge to make ucalls work. We have other
> MMIO devices (GIC distributor, for example) that work by chance since
> nothing conflicts with the constant GPAs we've selected in the tests.
> 
> I'd rather we go down the route of having an address allocator for the
> for both the VA and PA spaces to provide carveouts at runtime.

Aren't those two separate issues?  The PA, a.k.a. memslots space, can be solved
by allocating a dedicated memslot, i.e. doesn't need a carve.  At worst, collisions
will yield very explicit asserts, which IMO is better than whatever might go wrong
with a carve out.

> There's another issue with the new ucall implementation where identity
> mapping could stomp on a program segment that I'm fighting with right now
> which only further highlights the problems with our (mis)management of
> address spaces in selftests.

Oooh, this crud:

 	virt_pg_map(vm, mmio_gpa, mmio_gpa);

Yeah, that needs to be fixed.  But again, that's a separate issue, e.g. selftests
can allocate a virtual address and map the read-only memslot.