linux-kernel - Re: [PATCH 3/6] KVM: arm64: Add pre_fault

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Message-ID: <7d109638-3d26-443a-b24d-eb7a0059b80f@gmail.com>
Date: Mon, 29 Sep 2025 14:59:35 +0100
From: "Thomson, Jack" <jackabt.amazon@...il.com>
To: Oliver Upton <oliver.upton@...ux.dev>
Cc: maz@...nel.org, pbonzini@...hat.com, joey.gouly@....com,
 suzuki.poulose@....com, yuzenghui@...wei.com, catalin.marinas@....com,
 will@...nel.org, shuah@...nel.org, linux-arm-kernel@...ts.infradead.org,
 kvmarm@...ts.linux.dev, linux-kernel@...r.kernel.org,
 linux-kselftest@...r.kernel.org, isaku.yamahata@...el.com,
 roypat@...zon.co.uk, kalyazin@...zon.co.uk, jackabt@...zon.com
Subject: Re: [PATCH 3/6] KVM: arm64: Add pre_fault_memory implementation

Hi Oliver,

Thanks for reviewing!

On 11/09/2025 7:42 pm, Oliver Upton wrote:
> On Thu, Sep 11, 2025 at 02:46:45PM +0100, Jack Thomson wrote:
>> @@ -1607,7 +1611,7 @@ static int __user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>   			    struct kvm_s2_trans *nested,
>>   			    struct kvm_memory_slot *memslot,
>>   			    long *page_size, unsigned long hva,
>> -			    bool fault_is_perm)
>> +			    bool fault_is_perm, bool pre_fault)
>>   {
>>   	int ret = 0;
>>   	bool topup_memcache;
>> @@ -1631,10 +1635,13 @@ static int __user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>   	vm_flags_t vm_flags;
>>   	enum kvm_pgtable_walk_flags flags = KVM_PGTABLE_WALK_MEMABORT_FLAGS;
>>   
>> +	if (pre_fault)
>> +		flags |= KVM_PGTABLE_WALK_PRE_FAULT;
>> +
>>   	if (fault_is_perm)
>>   		fault_granule = kvm_vcpu_trap_get_perm_fault_granule(vcpu);
>> -	write_fault = kvm_is_write_fault(vcpu);
>> -	exec_fault = kvm_vcpu_trap_is_exec_fault(vcpu);
>> +	write_fault = !pre_fault && kvm_is_write_fault(vcpu);
>> +	exec_fault = !pre_fault && kvm_vcpu_trap_is_exec_fault(vcpu);
> 
> I'm not a fan of this. While user_mem_abort() is already a sloppy mess,
> one thing we could reliably assume is the presence of a valid fault
> context. Now we need to remember to special-case our interpretation of a
> fault on whether or not we're getting invoked for a pre-fault.
> 
> I'd rather see the pre-fault infrastructure compose a synthetic fault
> context (HPFAR_EL2, ESR_EL2, etc.). It places the complexity where it
> belongs and the rest of the abort handling code should 'just work'.
> 

Agreed, it looks much better with the synthetic abort. Is this the
approach you had in mind?

+long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
+				    struct kvm_pre_fault_memory *range)
+{
+	int ret, idx;
+	hva_t hva;
+	phys_addr_t end;
+	u64 esr, hpfar;
+	struct kvm_memory_slot *memslot;
+	struct kvm_vcpu_fault_info *fault_info;
+
+	long page_size = PAGE_SIZE;
+	phys_addr_t ipa = range->gpa;
+	gfn_t gfn = gpa_to_gfn(range->gpa);
+
+	idx = srcu_read_lock(&vcpu->kvm->srcu);
+
+	if (ipa >= kvm_phys_size(vcpu->arch.hw_mmu)) {
+		ret = -ENOENT;
+		goto out_unlock;
+	}
+
+	memslot = gfn_to_memslot(vcpu->kvm, gfn);
+	if (!memslot) {
+		ret = -ENOENT;
+		goto out_unlock;
+	}
+
+	fault_info = &vcpu->arch.fault;
+
+	esr = fault_info->esr_el2;
+	hpfar = fault_info->hpfar_el2;
+
+	fault_info->esr_el2 = ESR_ELx_FSC_ACCESS_L(KVM_PGTABLE_LAST_LEVEL);
+	fault_info->hpfar_el2 = HPFAR_EL2_NS |
+		((ipa >> (12 - HPFAR_EL2_FIPA_SHIFT)) & HPFAR_EL2_FIPA_MASK);
+
+	if (kvm_slot_has_gmem(memslot)) {
+		ret = gmem_abort(vcpu, ipa, NULL, memslot, false);
+	} else {
+		hva = gfn_to_hva_memslot_prot(memslot, gfn, NULL);
+		if (kvm_is_error_hva(hva)) {
+			ret = -EFAULT;
+			goto out;
+		}
+		ret = user_mem_abort(vcpu, ipa, NULL, memslot, &page_size, hva,
+				     false);
+	}
+
+	if (ret < 0)
+		goto out;
+
+	end = (range->gpa & ~(page_size - 1)) + page_size;
+	ret = min(range->size, end - range->gpa);
+
+out:
+	fault_info->esr_el2 = esr;
+	fault_info->hpfar_el2 = hpfar;
+out_unlock:
+	srcu_read_unlock(&vcpu->kvm->srcu, idx);
+	return ret;
+}

>> +long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
>> +				    struct kvm_pre_fault_memory *range)
>> +{
>> +	int r;
>> +	hva_t hva;
>> +	phys_addr_t end;
>> +	long page_size;
>> +	struct kvm_memory_slot *memslot;
>> +	phys_addr_t ipa = range->gpa;
>> +	gfn_t gfn = gpa_to_gfn(range->gpa);
>> +
>> +	while (true) {
>> +		page_size = PAGE_SIZE;
>> +		memslot = gfn_to_memslot(vcpu->kvm, gfn);
>> +		if (!memslot)
>> +			return -ENOENT;
>> +
>> +		if (kvm_slot_has_gmem(memslot)) {
>> +			r = __gmem_abort(vcpu, ipa, NULL, memslot, false, true);
>> +		} else {
>> +			hva = gfn_to_hva_memslot_prot(memslot, gfn, NULL);
>> +			if (kvm_is_error_hva(hva))
>> +				return -EFAULT;
>> +			r = __user_mem_abort(vcpu, ipa, NULL, memslot, &page_size, hva, false,
>> +					     true);
>> +		}
>> +
>> +		if (r != -EAGAIN)
>> +			break;
>> +
>> +		if (signal_pending(current))
>> +			return -EINTR;
>> +
>> +		if (kvm_check_request(KVM_REQ_VM_DEAD, vcpu))
>> +			return -EIO;
>> +
>> +		cond_resched();
>> +	};
> 
> Why do we need another retry loop? Looks like we've already got one in
> the arch-generic code.
>

Good point thanks, I've removed that now.

> 
> Thanks,
> Oliver

Thanks,
Jack