Re: [PATCH 10/11] KVM: arm64: CCA: Support runtime faulting of memory

[Suzuki K Poulose <suzuki.poulose@arm.com>]

Hi Fuad

Thanks for sharing the pVM gmem details, some more questions below.

On 09/11/2026 07:40, Fuad Tabba wrote:
> Hi Suzuki, Steve,
> 
> On Thu, 05 Nov 2026 17:30:16 +0000, Suzuki K Poulose
> <suzuki.poulose@arm.com> wrote:
> [...]
>>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> [...]
>>> @@ -1816,13 +1828,28 @@ static int gmem_abort(const struct kvm_s2_fault_desc *s2fd,
>>>          /* Pairs with the smp_wmb() in kvm_mmu_invalidate_end(). */
>>>          smp_rmb();
>>>
>>> +        if (kvm_vm_is_realm(kvm)) {
>>> +                is_realm_private = kvm_realm_is_private_address(&kvm->arch.realm,
>>> +                                                                s2fd->fault_ipa);
>>> +
>>> +                if (is_realm_private != kvm_gmem_is_private_gfn(kvm, canonical_gfn)) {
> [...]
>> I am wondering there is a way to make it a bit more generic. The issue
>> is, only Realm has private memory with guest_memfd for now. Don't know
>> the pKVM plans for it.
> 
> The plan is for protected pKVM VMs to be backed by guest_memfd too,
> which would also hold each page's private/shared state [1]. So
> kvm_arch_has_private_mem() will be true for them, and this comparison
> applies as is. Only how the expected state is computed differs: for a
> pVM it comes from the per-page share state rather than an IPA bit.

When you say, per-page, you mean per-IPA or per "backing page" ? i.e.,
can pKVM do something like :

    kvm_ipa_is_private(kvm, ipa) ?

(similar to Realm) or does it need the struct page backing the "ipa" ?

> 
> Steve, could you gate the guest_memfd checks on
> kvm_arch_has_private_mem(kvm) rather than kvm_vm_is_realm(), this one
> and the memslot check you proposed for 02/11? Only the
> kvm_realm_is_private_address() and realm_map_ipa() calls are
> realm-specific, and they could be VM_REALM cases of a switch on
> vm_flavor, as Marc asked for on the v21 memslot check [2]. The macro
> would then only widen to protected VMs when pKVM's guest_memfd support

Makes sense.

Suzuki

> lands.
> 
> Cheers,
> /fuad
> 
> [1] https://lore.kernel.org/all/CA+EHjTz+WgXEW5RTxXf4oGeEb1Tsm5Ha4nOc4S1jBcp7jxP4+A@mail.gmail.com/
> [2] https://lore.kernel.org/all/86mrsw2jrn.wl-maz@kernel.org/