linux-kernel - Re: [PATCH v7 34/45] kvm: rme: Hide KVM_CAP_READONLY

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Message-ID: <d254a8ea-0f02-4826-9af3-4a288efcc90c@arm.com>
Date: Mon, 7 Apr 2025 17:34:52 +0100
From: Steven Price <steven.price@....com>
To: Gavin Shan <gshan@...hat.com>, kvm@...r.kernel.org, kvmarm@...ts.linux.dev
Cc: 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>,
 Shanker Donthineni <sdonthineni@...dia.com>, Alper Gun
 <alpergun@...gle.com>, "Aneesh Kumar K . V" <aneesh.kumar@...nel.org>
Subject: Re: [PATCH v7 34/45] kvm: rme: Hide KVM_CAP_READONLY_MEM for realm
 guests

On 04/03/2025 11:51, Gavin Shan wrote:
> On 2/14/25 2:14 AM, Steven Price wrote:
>> For protected memory read only isn't supported. While it may be possible
>> to support read only for unprotected memory, this isn't supported at the
>> present time.
>>
>> Signed-off-by: Steven Price <steven.price@....com>
>> ---
>>   arch/arm64/kvm/arm.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
> 
> It's worthy to explain why KVM_CAP_READONLY_MEM isn't supported and its
> negative impact. It's something to be done in the future if I'm correct.

I'll add to the commit message:

    Note that this does mean that e.g. ROM (or flash) data cannot be
    emulated correctly by the VMM.

> From QEMU's perspective, all ROM data, which is populated by it, can
> be written. It conflicts to the natural limit: all ROM data should be
> read-only.

Yes this is my understanding of the main impact. I'm not sure how useful
(shared) ROM/flash emulation is. It can certainly be added in the future
if needed. Protected read-only memory I don't believe is useful - the
only sane response I can see from a write fault in that case is killing
the guest.

Thanks,
Steve

> QEMU
> ====
> rom_add_blob
>   rom_set_mr
>     memory_region_set_readonly
>       memory_region_transaction_commit
>         kvm_region_commit
>           kvm_set_phys_mem
>             kvm_mem_flags                                    // flag
> KVM_MEM_READONLY is missed
>             kvm_set_user_memory_region
>               kvm_vm_ioctl(KVM_SET_USER_MEMORY_REGION2)
> 
> non-secure host
> ===============
> rec_exit_sync_dabt
>   kvm_handle_guest_abort
>     user_mem_abort
>       __kvm_faultin_pfn                       // writable == true
>         realm_map_ipa
>           WARN_ON(!(prot & KVM_PGTABLE_PROT_W)
> 
> non-secure host
> ===============
> kvm_realm_enable_cap(KVM_CAP_ARM_RME_POPULATE_REALM)
>   kvm_populate_realm
>     __kvm_faultin_pfn                      // writable == true
>       realm_create_protected_data_page
> 
>> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
>> index 1f3674e95f03..0f1d65f87e2b 100644
>> --- a/arch/arm64/kvm/arm.c
>> +++ b/arch/arm64/kvm/arm.c
>> @@ -348,7 +348,6 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm,
>> long ext)
>>       case KVM_CAP_ONE_REG:
>>       case KVM_CAP_ARM_PSCI:
>>       case KVM_CAP_ARM_PSCI_0_2:
>> -    case KVM_CAP_READONLY_MEM:
>>       case KVM_CAP_MP_STATE:
>>       case KVM_CAP_IMMEDIATE_EXIT:
>>       case KVM_CAP_VCPU_EVENTS:
>> @@ -362,6 +361,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm,
>> long ext)
>>       case KVM_CAP_COUNTER_OFFSET:
>>           r = 1;
>>           break;
>> +    case KVM_CAP_READONLY_MEM:
>>       case KVM_CAP_SET_GUEST_DEBUG:
>>           r = !kvm_is_realm(kvm);
>>           break;
> 
> Thanks,
> Gavin
>