On Aug 21, 2014, at 2:56 PM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> Il 21/08/2014 10:05, Wanpeng Li ha scritto:
>> Hi Nadav,
>> On Wed, Aug 20, 2014 at 03:11:51PM +0300, Nadav Amit wrote:
>>> Currently, when an msr is updated using kvm_set_shared_msr the masked bits are
>>> zeroed. This behavior is currently valid since the only MSR with partial mask
>>
>> Why zeroed? vmx_vcpu_setup() set all mask to -1ull.
>
> He meant they are passed as zero in the WRMSR but actually they're not
> zeroed. They're set to the value that is passed to kvm_set_shared_msr,
> and this value is massaged elsewhere to do mix guest and host bugs. See
> update_transition_efer.
>
> So I'm removing this patch, it's wrong.
I stand corrected - they are massaged in update_transition_efer.
The question is whether this massaging is specific to EFER, or a general one.
Currently update_transition_efer does:
guest_efer &= ~ignore_bits;
guest_efer |= host_efer & ignore_bits;
vmx->guest_msrs[efer_offset].data = guest_efer;
I think this is a general behaviour - taking the masked bits from the host, and the rest from the guest. Therefore, it makes sense to put this logic into kvm_set_shared_msr.
I understand the EFER is currently the only MSR which is only partially masked. Nonetheless, kvm_set_shared_msr can be useful for other purposes.
Nadav
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