linux-kernel - Re: [PATCH v4 21/43] arm64: RME: Runtime faulting of memory

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Message-ID: <bdeb29af-b833-402d-b0b2-0db02335a0c3@arm.com>
Date: Mon, 2 Sep 2024 16:34:19 +0100
From: Steven Price <steven.price@....com>
To: Matias Ezequiel Vara Larsen <mvaralar@...hat.com>
Cc: kvm@...r.kernel.org, kvmarm@...ts.linux.dev,
 Catalin Marinas <catalin.marinas@....com>, Marc Zyngier <maz@...nel.org>,
 Will Deacon <will@...nel.org>, James Morse <james.morse@....com>,
 Oliver Upton <oliver.upton@...ux.dev>,
 Suzuki K Poulose <suzuki.poulose@....com>, Zenghui Yu
 <yuzenghui@...wei.com>, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, Joey Gouly <joey.gouly@....com>,
 Alexandru Elisei <alexandru.elisei@....com>,
 Christoffer Dall <christoffer.dall@....com>, Fuad Tabba <tabba@...gle.com>,
 linux-coco@...ts.linux.dev,
 Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>,
 Gavin Shan <gshan@...hat.com>, Shanker Donthineni <sdonthineni@...dia.com>,
 Alper Gun <alpergun@...gle.com>
Subject: Re: [PATCH v4 21/43] arm64: RME: Runtime faulting of memory

On 02/09/2024 14:25, Matias Ezequiel Vara Larsen wrote:
> Hello Steven, 
> 
> On Wed, Aug 21, 2024 at 04:38:22PM +0100, Steven Price wrote:

...

>> +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;
>> +	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 I understand correctly, `kvm_prepare_memory_fault_exit()` ends up
> returning to the VMM with the KVM_EXIT_MEMORY_FAULT exit reason. The
> documentation says (https://docs.kernel.org/virt/kvm/api.html#kvm-run):
> 
> "Note! KVM_EXIT_MEMORY_FAULT is unique among all KVM exit reasons in that
> it accompanies a return code of ‘-1’, not ‘0’! errno will always be set
> to EFAULT or EHWPOISON when KVM exits with KVM_EXIT_MEMORY_FAULT,
> userspace should assume kvm_run.exit_reason is stale/undefined for all
> other error numbers."
> 
> Shall the return code be different for KVM_EXIT_MEMORY_FAULT?
> 
> Thanks, Matias.

Ah, good spot - I've no idea why KVM_EXIT_MEMORY_FAULT is special in
this regard, but yes I guess we should be returning -EFAULT here.

Thanks,
Steve