Messages in this thread | | | Subject | Re: [PATCH v11 5/6] KVM: arm64: ioctl to fetch/store tags in a guest | From | Steven Price <> | Date | Fri, 7 May 2021 10:44:33 +0100 |
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On 04/05/2021 18:44, Catalin Marinas wrote: > On Thu, Apr 29, 2021 at 05:06:07PM +0100, Steven Price wrote: >> On 27/04/2021 18:58, Catalin Marinas wrote: >>> On Fri, Apr 16, 2021 at 04:43:08PM +0100, Steven Price wrote: >>>> diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h >>>> index 24223adae150..2b85a047c37d 100644 >>>> --- a/arch/arm64/include/uapi/asm/kvm.h >>>> +++ b/arch/arm64/include/uapi/asm/kvm.h >>>> @@ -184,6 +184,20 @@ struct kvm_vcpu_events { >>>> __u32 reserved[12]; >>>> }; >>>> +struct kvm_arm_copy_mte_tags { >>>> + __u64 guest_ipa; >>>> + __u64 length; >>>> + union { >>>> + void __user *addr; >>>> + __u64 padding; >>>> + }; >>>> + __u64 flags; >>>> + __u64 reserved[2]; >>>> +}; > [...] >>> Maybe add the two reserved >>> values to the union in case we want to store something else in the >>> future. >> >> I'm not sure what you mean here. What would the reserved fields be unioned >> with? And surely they are no longer reserved in that case? > > In case you want to keep the structure size the same for future > expansion and the expansion only happens via the union, you'd add some > padding in there just in case. We do this for struct siginfo with an > _si_pad[] array in the union. >
Ah I see what you mean. In this case "padding" is just a sizer to ensure that flags is always the same alignment - it's not intended to be used. As I noted previously though it's completely pointless as this only on arm64 and even 32 bit Arm would naturally align the following __u64.
reserved[] is for expansion and I guess we could have a union over the whole struct (like siginfo) but I think it's generally clearer to just spell out the reserved fields at the end of the struct.
TLDR; the union will be gone along with "padding" in the next version. "reserved" remains at the end of the struct for future use.
Thanks,
Steve
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