Re: [patch 3/5] KVM: hypercall batching (v2)

[Avi Kivity <avi@qumranet.com>]

Marcelo Tosatti wrote:
> On Thu, Feb 21, 2008 at 05:52:25PM +0200, Avi Kivity wrote:
>   
>> Marcelo Tosatti wrote:
>>     
>>> Batch pte updates and tlb flushes in lazy MMU mode.
>>>
>>> v1->v2:
>>> - report individual hypercall error code, have multicall return number of 
>>> processed entries.
>>> - cover entire multicall duration with slots_lock instead of 
>>> acquiring/reacquiring.
>>>  
>>>       
>> But not all hypercalls want slots_lock.
>>     
>
> But slots_lock is required for kvm_read_guest() (and write_guest()). So
> even if the underlying hypercall handling does not require slots_lock,
> it still makes sense.
>
>   

Right.  I guess it is cheap enough so we can keep this.

>> I suggested earlier switching to a "multiple mmu operation" hypercall 
>> (and not have individual mmu hypercalls). What do think about that?
>>     
>
> We need to support different MMU operations in the same multicall, for
> example normal pte writes, masked pte updates and tlb flushes. So an
> array of
>
> "operation, parameters"
>
> is required.
>
>   
>> I think hypercalls will be quite diverse in the future and batching them 
>> will not make sense always.
>>     
>
> I agree. While the infrastructure is generic allowing batching of any
> kind of hypercall, we explicitly select which ones can be batched (in
> the guest):
>
> static int can_defer_hypercall(struct kvm_para_state *state, unsigned int nr)
> {
>         if (state->mode == PARAVIRT_LAZY_MMU) {
>                 switch (nr) {
>                 case KVM_HYPERCALL_MMU_WRITE:
>                 case KVM_HYPERCALL_FLUSH_TLB:
>                         return 1;
>                 }
>         }
>         return 0;
> }
>
> Perhaps you want to move that enforcement to the host.
>
> This allows batching of future hypercalls (if appropriate) to be easy.
>
>   

I'm still uneasy about it, though I have no rational reasons left now.

Oh, there is one: with a MMU_OP hypercall you can take the mmu spinlock 
once per batch (dropping it once in a while to let another vcpu make 
progress or to inject an interrupt).

-- 
error compiling committee.c: too many arguments to function


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