[PATCH 13/15] KVM: ARM: Handle guest faults in KVM

[Catalin Marinas]

On 25 September 2012 13:38, Christoffer Dall
<c.dall at virtualopensystems.com> wrote:
>>> +
>>> +     /*
>>> +      * If this is a write fault (think COW) we need to make sure the
>>> +      * existing page, which other CPUs might still read, doesn't go
>>> away
>>> +      * from under us, by calling gfn_to_pfn_prot(write_fault=true).
>>> +      * Therefore, we call gfn_to_pfn_prot(write_fault=false), which
>>> will
>>> +      * pin the existing page, then we get a new page for the user space
>>> +      * pte and map this in the stage-2 table where we also make sure to
>>> +      * flush the TLB for the VM, if there was an existing entry (the
>>> entry
>>> +      * was updated setting the write flag to the potentially new page).
>>> +      */
>>> +     if (fault_status == FSC_PERM) {
>>> +             pfn_existing = gfn_to_pfn_prot(vcpu->kvm, gfn, false, NULL);
>>> +             if (is_error_pfn(pfn_existing))
>>> +                     return -EFAULT;
>>> +     }
>>> +
>>> +     pfn = gfn_to_pfn_prot(vcpu->kvm, gfn, write_fault, &writable);
>>> +     if (is_error_pfn(pfn)) {
>>> +             ret = -EFAULT;
>>> +             goto out_put_existing;
>>> +     }
>>> +
>>> +     /* We need minimum second+third level pages */
>>> +     ret = mmu_topup_memory_cache(memcache, 2, KVM_NR_MEM_OBJS);
>>> +     if (ret)
>>> +             goto out;
>>> +     new_pte = pfn_pte(pfn, PAGE_KVM_GUEST);
>>> +     if (writable)
>>> +             pte_val(new_pte) |= L_PTE2_WRITE;
>>> +     coherent_icache_guest_page(vcpu->kvm, gfn);
>>
>> why don't you flush icache only when guest has mapped executable page
>> as __sync_icache_dcache function does currently?
>
> because we don't know if the guest will map the page executable. The
> guest may read the page through a normal load, which causes the fault,
> and subsequently execute it (even possible through different guest
> mappings). The only way to see this happening would be to mark all
> pages as non-executable and catch the fault when it occurs -
> unfortunately the HPFAR which gives us the IPA is not populated on
> execute never faults, so we would have to translate the PC's va to ipa
> using cp15 functionality when this happens, which is then also racy
> with other CPUs.

I think you can avoid the race in the stage 2 XN case. In the Hyp
exception handler entered because of a stage 2 XN bit you can get the
IPA via the CP15 ATS1CPR and PAR registers. If the address translation
failed because the same guest running on other CPU changed the stage 1
page table, you can simply return to the guest rather than switching
to host with incomplete information. The guest may handle its own
stage 1 fault and eventually trigger another stage 2 permission and
Hyp will try the address translation again. That's a very rare
situation, so just returning without handling it would not cause any
performance issues.

-- 
Catalin