Re: [PATCH 9/9] KVM: selftests: Add option to run dirty_log_perf_test vCPUs in L2

[Peter Xu <peterx@redhat.com>]

On Mon, May 16, 2022 at 03:34:28PM -0700, David Matlack wrote:
> On Mon, May 16, 2022 at 3:17 PM Peter Xu <peterx@redhat.com> wrote:
> >
> > On Fri, Apr 29, 2022 at 06:39:35PM +0000, David Matlack wrote:
> > > +static void perf_test_l1_guest_code(struct vmx_pages *vmx, uint64_t vcpu_id)
> > > +{
> > > +#define L2_GUEST_STACK_SIZE 64
> > > +     unsigned long l2_guest_stack[L2_GUEST_STACK_SIZE];
> > > +     unsigned long *rsp;
> > > +
> > > +     GUEST_ASSERT(vmx->vmcs_gpa);
> > > +     GUEST_ASSERT(prepare_for_vmx_operation(vmx));
> > > +     GUEST_ASSERT(load_vmcs(vmx));
> > > +     GUEST_ASSERT(ept_1g_pages_supported());
> > > +
> > > +     rsp = &l2_guest_stack[L2_GUEST_STACK_SIZE - 1];
> > > +     *rsp = vcpu_id;
> > > +     prepare_vmcs(vmx, perf_test_l2_guest_entry, rsp);
> >
> > Just to purely ask: is this setting the same stack pointer to all the
> > vcpus?
> 
> No, but I understand the confusion since typically selftests use
> symbols like "l2_guest_code" that are global. But "l2_guest_stack" is
> actually a local variable so it will be allocated on the stack. Each
> vCPU runs on a separate stack, so they will each run with their own
> "l2_guest_stack".

Ahh that's correct!

> 
> >
> > > +
> > > +     GUEST_ASSERT(!vmlaunch());
> > > +     GUEST_ASSERT(vmreadz(VM_EXIT_REASON) == EXIT_REASON_VMCALL);
> > > +     GUEST_DONE();
> > > +}
> >
> > [...]
> >
> > > +/* Identity map the entire guest physical address space with 1GiB Pages. */
> > > +void nested_map_all_1g(struct vmx_pages *vmx, struct kvm_vm *vm)
> > > +{
> > > +     __nested_map(vmx, vm, 0, 0, vm->max_gfn << vm->page_shift, PG_LEVEL_1G);
> > > +}
> >
> > Could max_gfn be large?  Could it consumes a bunch of pages even if mapping
> > 1G only?
> 
> Since the selftests only support 4-level EPT, this will use at most
> 513 pages. If we add support for 5-level EPT we may need to revisit
> this approach.

It's just that AFAICT vm_alloc_page_table() is fetching from slot 0 for all
kinds of pgtables including EPT.  I'm not sure whether there can be some
failures conditionally with this because when creating the vm we're not
aware of this consumption, so maybe we'd reserve the pages somehow so that
we'll be sure to have those pages at least?

-- 
Peter Xu