Subject: Re: [RFC PATCH v12 18/33] KVM: x86/mmu: Handle page fault for private memory

From: Binbin Wu <[email-masked]>
Date: Thu, 21 Sep 2023 13:51:34 +0800

On 9/15/2023 10:26 PM, Sean Christopherson wrote:
> On Fri, Sep 15, 2023, Yan Zhao wrote:
>> On Wed, Sep 13, 2023 at 06:55:16PM -0700, Sean Christopherson wrote:
>> ....
>>> +static void kvm_mmu_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
>>> +					      struct kvm_page_fault *fault)
>>> +{
>>> +	kvm_prepare_memory_fault_exit(vcpu, fault->gfn << PAGE_SHIFT,
>>> +				      PAGE_SIZE, fault->write, fault->exec,
>>> +				      fault->is_private);
>>> +}
>>> +
>>> +static int kvm_faultin_pfn_private(struct kvm_vcpu *vcpu,
>>> +				   struct kvm_page_fault *fault)
>>> +{
>>> +	int max_order, r;
>>> +
>>> +	if (!kvm_slot_can_be_private(fault->slot)) {
>>> +		kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
>>> +		return -EFAULT;
>>> +	}
>>> +
>>> +	r = kvm_gmem_get_pfn(vcpu->kvm, fault->slot, fault->gfn, &fault->pfn,
>>> +			     &max_order);
>>> +	if (r) {
>>> +		kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
>>> +		return r;
>>> +	}
>>> +
>>> +	fault->max_level = min(kvm_max_level_for_order(max_order),
>>> +			       fault->max_level);
>>> +	fault->map_writable = !(fault->slot->flags & KVM_MEM_READONLY);
>>> +
>>> +	return RET_PF_CONTINUE;
>>> +}
>>> +
>>>   static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>>>   {
>>>   	struct kvm_memory_slot *slot = fault->slot;
>>> @@ -4293,6 +4356,14 @@ static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
>>>   			return RET_PF_EMULATE;
>>>   	}
>>>   
>>> +	if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
>> In patch 21,
>> fault->is_private is set as:
>> 	".is_private = kvm_mem_is_private(vcpu->kvm, cr2_or_gpa >> PAGE_SHIFT)",
>> then, the inequality here means memory attribute has been updated after
>> last check.
>> So, why an exit to user space for converting is required instead of a mere retry?
>>
>> Or, is it because how .is_private is assigned in patch 21 is subjected to change
>> in future?
> This.  Retrying on SNP or TDX would hang the guest.  I suppose we could special
> case VMs where .is_private is derived from the memory attributes, but the
> SW_PROTECTED_VM type is primary a development vehicle at this point.  I'd like to
> have it mimic SNP/TDX as much as possible; performance is a secondary concern.
So when .is_private is derived from the memory attributes, and if I 
didn't miss
anything, there is no explicit conversion mechanism introduced yet so 
far, does
it mean for pure sw-protected VM (withouth SNP/TDX), the page fault will be
handled according to the memory attributes setup by host/user vmm, no 
implicit
conversion will be triggered, right?

>
> E.g. userspace needs to be prepared for "spurious" exits due to races on SNP and
> TDX, which this can theoretically exercise.  Though the window is quite small so
> I doubt that'll actually happen in practice; which of course also makes it less
> important to retry instead of exiting.