Re: [PATCH v2 21/43] arm64: RME: Runtime faulting of memory

Steven Price
2024-05-31T16:04:00Z

On 25/04/2024 11:43, Fuad Tabba wrote:
> Hi,

Hi,

Thanks for the review. Sorry I didn't respond earlier.

> On Fri, Apr 12, 2024 at 9:44 AM Steven Price <steven.price@arm.com> wrote:
>>
<snip>
>> +static int private_memslot_fault(struct kvm_vcpu *vcpu,
>> + phys_addr_t fault_ipa,
>> + struct kvm_memory_slot *memslot)
>> +{
>> + struct kvm *kvm = vcpu->kvm;
>> + gpa_t gpa_stolen_mask = kvm_gpa_stolen_bits(kvm);
>> + gfn_t gfn = (fault_ipa & ~gpa_stolen_mask) >> PAGE_SHIFT;
>> + bool is_priv_gfn = !((fault_ipa & gpa_stolen_mask) == gpa_stolen_mask);
>> + bool priv_exists = kvm_mem_is_private(kvm, gfn);
>> + struct kvm_mmu_memory_cache *memcache = &vcpu->arch.mmu_page_cache;
>> + int order;
>> + kvm_pfn_t pfn;
>> + int ret;
>> +
>> + if (priv_exists != is_priv_gfn) {
>> + kvm_prepare_memory_fault_exit(vcpu,
>> + fault_ipa & ~gpa_stolen_mask,
>> + PAGE_SIZE,
>> + kvm_is_write_fault(vcpu),
>> + false, is_priv_gfn);
>> +
>> + return 0;
>> + }
>> +
>> + if (!is_priv_gfn) {
>> + /* Not a private mapping, handling normally */
>> + return -EAGAIN;
>> + }
>> +
>> + if (kvm_gmem_get_pfn(kvm, memslot, gfn, &pfn, &order))
>> + return 1; /* Retry */
>
> You don't need to pass a variable to hold the order if you don't need
> it. You can pass NULL.

Ah, good point - that simplifies things.

> I am also confused about the return, why do you return 1 regardless of
> the reason kvm_gmem_get_pfn() fails?

Thinking about this, I don't think we actually expect kvm_gmem_get_pfn()
to fail, so it's actually more appropriate to just pass return any error
value.

>> + ret = kvm_mmu_topup_memory_cache(memcache,
>> + kvm_mmu_cache_min_pages(vcpu->arch.hw_mmu));
>> + if (ret)
>> + return ret;
>
> If this fails you should release the page you got earlier (e.g.,
> kvm_release_pfn_clean()), or you could move it before
> kvm_gmem_get_pfn().

Good point, however...

>> + /* FIXME: Should be able to use bigger than PAGE_SIZE mappings */
>> + ret = realm_map_ipa(kvm, fault_ipa, pfn, PAGE_SIZE, KVM_PGTABLE_PROT_W,
>> + memcache);
>> + if (!ret)
>> + return 1; /* Handled */
>
> Should also release the page if it fails. Speaking of which,
> where/when do you eventually release the page?

... I messed this up ;) It seems I'm managing to leak all guestmem
pages. I'm not sure what I was thinking but I think I'd got it into my
head guestmem wasn't reference counting the pages. I'll fix this up in
the next version.

Thanks,

Steve