Re: [PATCH v2 1/3] kvm: svm: Add support for additional SVM NPF error codes

[Paolo Bonzini <pbonzini@redhat.com>]

On 31/07/2017 15:30, Brijesh Singh wrote:
> Hi Paolo,
> 
> On 07/27/2017 11:27 AM, Paolo Bonzini wrote:
>> On 23/11/2016 18:01, Brijesh Singh wrote:
>>>   +    /*
>>> +     * Before emulating the instruction, check if the error code
>>> +     * was due to a RO violation while translating the guest page.
>>> +     * This can occur when using nested virtualization with nested
>>> +     * paging in both guests. If true, we simply unprotect the page
>>> +     * and resume the guest.
>>> +     *
>>> +     * Note: AMD only (since it supports the PFERR_GUEST_PAGE_MASK used
>>> +     *       in PFERR_NEXT_GUEST_PAGE)
>>> +     */
>>> +    if (error_code == PFERR_NESTED_GUEST_PAGE) {
>>> +        kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(cr2));
>>> +        return 1;
>>> +    }
>>
>>
>> What happens if L1 is mapping some memory that is read only in L0?  That
>> is, the L1 nested page tables make it read-write, but the L0 shadow
>> nested page tables make it read-only.
>>
>> Accessing it would cause an NPF, and then my guess is that the L1 guest
>> would loop on the failing instruction instead of just dropping the write.
>>
> 
> 
> Not sure if I am able to follow your use case. Could you please explain me
> in bit detail.
> 
> The purpose of the code above was really for when we resume from the L2 guest
> back to the L1 guest. The L1 page tables are marked RO when in the L2 guest
> (for shadow paging) as I recall, so when we come back to the L1 guest, it can
> get a fault since its page tables are not marked writeable at L0 as they
> need to be.

There can be different cases where an L0->L2 shadow nested page table is
marked read only, in particular when a page is read only in L1's nested
page tables.  If such a page is accessed by L2 while walking page tables
it will cause a nested page fault (page table walks are write accesses).
 However, after kvm_mmu_unprotect_page you will get another page fault,
and again in an endless stream.

Instead, emulation would have caused a nested page fault vmexit, I think.

Paolo