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@xxxxxxx>
>> ---
>> 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
>
