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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 04/27/2018 11:52 AM, Jan Beulich wrote:
>>>> 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?


Something like

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 ( new_state == vmcb_sync_to_cpu )
    {
        if (v->arch.hvm_svm.vmcb_sync_state == vmcb_needs_vmload )
        {     
            svm_vmload(vmcb);
            v->arch.hvm_svm.vmcb_sync_state = vmcb_in_sync;
        }
        return;
    }

    if ( arch_svm->vmcb_sync_state == vmcb_needs_vmsave )
        svm_vmsave(arch_svm->vmcb);
 
    if ( arch_svm->vmcb_sync_state != vmcb_needs_vmload )
        arch_svm->vmcb_sync_state = new_state;
 }


-boris


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