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SubjectRe: [PATCH v6 1/7] KVM: x86: Deflect unknown MSR accesses to user space
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On 16.09.20 19:08, Sean Christopherson wrote:
>
> On Wed, Sep 16, 2020 at 11:31:30AM +0200, Alexander Graf wrote:
>> On 03.09.20 21:27, Aaron Lewis wrote:
>>>> @@ -412,6 +414,15 @@ struct kvm_run {
>>>> __u64 esr_iss;
>>>> __u64 fault_ipa;
>>>> } arm_nisv;
>>>> + /* KVM_EXIT_X86_RDMSR / KVM_EXIT_X86_WRMSR */
>>>> + struct {
>>>> + __u8 error; /* user -> kernel */
>>>> + __u8 pad[3];
>>>
>>> __u8 pad[7] to maintain 8 byte alignment? unless we can get away with
>>> fewer bits for 'reason' and
>>> get them from 'pad'.
>>
>> Why would we need an 8 byte alignment here? I always thought natural u64
>> alignment on x86_64 was on 4 bytes?
>
> u64 will usually (always?) be 8 byte aligned by the compiler. "Natural"
> alignment means an object is aligned to its size. E.g. an 8-byte object
> can split a cache line if it's only aligned on a 4-byte boundary.

For some reason I always thought that x86_64 had a special hack that
allows u64s to be "naturally" aligned on a 32bit boundary. But I just
double checked what you said and indeed, gcc does pad it to an actual
natural boundary.

You never stop learning :).

In that case, it absolutely makes sense to make the padding explicit
(and pull it earlier)!


Alex




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