Messages in this thread | | | Subject | Re: [PATCH v6 1/7] KVM: x86: Deflect unknown MSR accesses to user space | From | Alexander Graf <> | Date | Wed, 16 Sep 2020 21:15:53 +0200 |
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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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