Re: [Xen-devel] [PATCH] SVM: re-work VMCB sync-ing

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 26.04.18 at 19:27, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 04/26/2018 11:55 AM, Jan Beulich wrote:
>>>>> On 26.04.18 at 17:20, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>> On 04/26/2018 09:33 AM, Jan Beulich wrote:
>>>>>> -static void svm_sync_vmcb(struct vcpu *v)
>>>>>> +static void svm_sync_vmcb(struct vcpu *v, enum vmcb_sync_state 
>>>>>> new_state)
>>>>>>  {
>>>>>>      struct arch_svm_struct *arch_svm = &v->arch.hvm_svm;
>>>>>>  
>>>>>> -    if ( arch_svm->vmcb_in_sync )
>>>>>> -        return;
>>>>>> -
>>>>>> -    arch_svm->vmcb_in_sync = 1;
>>>>>> +    if ( arch_svm->vmcb_sync_state == vmcb_needs_vmsave )
>>>>>> +        svm_vmsave(arch_svm->vmcb);
>>>>>>  
>>>>>> -    svm_vmsave(arch_svm->vmcb);
>>>>>> +    if ( arch_svm->vmcb_sync_state != vmcb_needs_vmload )
>>>>>> +        arch_svm->vmcb_sync_state = new_state;
>>>>> This is slightly awkward for a couple of reasons.  First, passing
>>>>> vmcb_in_sync in forget the fact that a vmload is needed.
>>>> Certainly not - that's the purpose of the if() around it.
>>>>
>>>>> In my patch, I introduced svm_sync_vmcb_for_update(), rather than
>>>>> requiring a parameter to be passed in.  I think this is a better API,
>>>>> and it shrinks the size of the patch.
>>>> I'm not convinced of the "better", and even less so of the "shrinks". But
>>>> I'll wait to see what the SVM maintainers say.
>>>
>>> I think a single function is better. In fact, I was wondering whether
>>> svm_vmload() could also be folded into svm_sync_vmcb() since it is also
>>> a syncing operation.
>> That doesn't look like it would produce a usable interface: How would
>> you envision the state transition to be specified by the caller? Right
>> now the intended new state gets passed in, but in your model
>> vmcb_in_sync could mean either vmload or vmsave is needed. The
>> two svm_vmload() uses right now would pass that value in addition
>> to the svm_sync_vmcb() calls already doing so. And the function
>> couldn't tell what to do from the current state (if it's
>> vmcb_needs_vmload, a load is only needed in the cases where
>> svm_vmload() is called right now). Adding a 3rd parameter or a
>> second enum 
> 
> I was thinking about another enum value, e.g. sync_to_cpu (and
> sync_to_vmcb replacing vmcb_needs_vmsave).
> 
> This will allow us to hide (v->arch.hvm_svm.vmcb_sync_state ==
> vmcb_needs_vmload) test.

I'm still not entirely clear how you want that to look like. At the example
of svm_get_segment_register() and svm_set_segment_register(), how
would the svm_sync_vmcb() calls look like? I.e. how do you distinguish
the sync_to_vmcb/transition-to-clean case from the
sync_to_vmcb/transition-to-dirty one?

Jan



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