Re: [PATCH v15 13/21] KVM: x86/mmu: Handle guest page faults for guest_memfd with shared memory

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 7/22/2025 12:47 AM, Sean Christopherson wrote:
> On Thu, Jul 17, 2025, Fuad Tabba wrote:
>> From: Ackerley Tng <ackerleytng@google.com>
>>
>> Update the KVM MMU fault handler to service guest page faults
>> for memory slots backed by guest_memfd with mmap support. For such
>> slots, the MMU must always fault in pages directly from guest_memfd,
>> bypassing the host's userspace_addr.
>>
>> This ensures that guest_memfd-backed memory is always handled through
>> the guest_memfd specific faulting path, regardless of whether it's for
>> private or non-private (shared) use cases.
>>
>> Additionally, rename kvm_mmu_faultin_pfn_private() to
>> kvm_mmu_faultin_pfn_gmem(), as this function is now used to fault in
>> pages from guest_memfd for both private and non-private memory,
>> accommodating the new use cases.
>>
>> Co-developed-by: David Hildenbrand <david@redhat.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> Signed-off-by: Ackerley Tng <ackerleytng@google.com>
>> Co-developed-by: Fuad Tabba <tabba@google.com>
>> Signed-off-by: Fuad Tabba <tabba@google.com>
>> ---
>>   arch/x86/kvm/mmu/mmu.c | 13 +++++++++----
>>   1 file changed, 9 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
>> index 94be15cde6da..ad5f337b496c 100644
>> --- a/arch/x86/kvm/mmu/mmu.c
>> +++ b/arch/x86/kvm/mmu/mmu.c
>> @@ -4511,8 +4511,8 @@ static void kvm_mmu_finish_page_fault(struct kvm_vcpu *vcpu,
>>   				 r == RET_PF_RETRY, fault->map_writable);
>>   }
>>   
>> -static int kvm_mmu_faultin_pfn_private(struct kvm_vcpu *vcpu,
>> -				       struct kvm_page_fault *fault)
>> +static int kvm_mmu_faultin_pfn_gmem(struct kvm_vcpu *vcpu,
>> +				    struct kvm_page_fault *fault)
>>   {
>>   	int max_order, r;
>>   
>> @@ -4536,13 +4536,18 @@ static int kvm_mmu_faultin_pfn_private(struct kvm_vcpu *vcpu,
>>   	return RET_PF_CONTINUE;
>>   }
>>   
>> +static bool fault_from_gmem(struct kvm_page_fault *fault)
> 
> Drop the helper.  It has exactly one caller, and it makes the code *harder* to
> read, e.g. raises the question of what "from gmem" even means.  If a separate
> series follows and needs/justifies this helper, then it can/should be added then.

there is another place requires the same check introduced by your
"KVM: x86/mmu: Extend guest_memfd's max mapping level to shared 
mappings" provided in [*]

[*] https://lore.kernel.org/kvm/aH7KghhsjaiIL3En@google.com/

---
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 1ff7582d5fae..2d1894ed1623 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c

@@ -3335,8 +3336,9 @@ int kvm_mmu_max_mapping_level(struct kvm *kvm, 
struct kvm_page_fault *fault,
         if (max_level == PG_LEVEL_4K)
                 return PG_LEVEL_4K;

-       if (is_private)
-               host_level = kvm_max_private_mapping_level(kvm, fault, 
slot, gfn);
+       if (is_private || kvm_memslot_is_gmem_only(slot))
+               host_level = kvm_gmem_max_mapping_level(kvm, fault, 
slot, gfn,
+                                                       is_private);
         else
                 host_level = host_pfn_mapping_level(kvm, gfn, slot);
         return min(host_level, max_level);

>> +{
>> +	return fault->is_private || kvm_memslot_is_gmem_only(fault->slot);
>> +}
>> +
>>   static int __kvm_mmu_faultin_pfn(struct kvm_vcpu *vcpu,
>>   				 struct kvm_page_fault *fault)
>>   {
>>   	unsigned int foll = fault->write ? FOLL_WRITE : 0;
>>   
>> -	if (fault->is_private)
>> -		return kvm_mmu_faultin_pfn_private(vcpu, fault);
>> +	if (fault_from_gmem(fault))
>> +		return kvm_mmu_faultin_pfn_gmem(vcpu, fault);
>>   
>>   	foll |= FOLL_NOWAIT;
>>   	fault->pfn = __kvm_faultin_pfn(fault->slot, fault->gfn, foll,
>> -- 
>> 2.50.0.727.gbf7dc18ff4-goog
>>