Re: [patch] kvm: make cr3 loading more robust

Ingo Molnar
2007-01-03T11:59:11Z

* Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org> wrote:

> >[ Avi: this patch needs kvm_mmu_get_page() NULL return to not be ignored
> > by mmu_alloc_roots()/nonpaging_init_context()/paging_new_cr3()/etc.
> > and passed further down - i assume you are working on those
> > issues already, or should i fix those too? ]
>
> kvm_mmu_get_page() cannot return NULL (before this patch), as we
> ensure there are free pages before entry to page_fault.

hm, where do we ensure that? kvm_mmu_get_page() calls
kvm_mmu_alloc_page(), which might return NULL:

static struct kvm_mmu_page *kvm_mmu_alloc_page(struct kvm_vcpu *vcpu,
u64 *parent_pte)
{
struct kvm_mmu_page *page;

if (list_empty(&vcpu->free_pages))
return NULL;

and i dont see where we ensure that ->free_pages is not empty. The
relevant callchain would be kvm_vmx_return() -> handle_cr() -> set_cr3()
-> paging_new_cr3() -> mmu_alloc_roots() -> kvm_mmu_get_page().

is there some accounting that makes a NULL pointer return impossible?
The only one i can see is the direct quering of vcpu->free_pages or the
querying of n_free_mmu_pages, but neither seems to be done in this
codepath. (but i might have missed it)

Ingo