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

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Ingo Molnar wrote:
> * 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)
>
>   

The page fault path does this:

static inline int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t gva,
                     u32 error_code)
{
    if (unlikely(vcpu->kvm->n_free_mmu_pages < KVM_MIN_FREE_MMU_PAGES))
        kvm_mmu_free_some_pages(vcpu);
    return vcpu->mmu.page_fault(vcpu, gva, error_code);
}



The context switch paths need to do the same.


-- 
error compiling committee.c: too many arguments to function


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