Re: Paravirt KVM capabilities

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Ingo Molnar wrote:
> * Ulrich Drepper <drepper-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
>
>   
>> Ingo Molnar wrote:
>>     
>>> i really really think KVM and lhype should merge, creating KVM/HVM 
>>> (Hardware Virtual Machine) and KVM/LL (Linux on Linux).
>>>       
>> This is only sufficient if either KVM with paravirt Linux kernels has 
>> no performance penalty or lhype becomes able to execute paravirt Linux 
>> kernels.
>>     
>
> yes, both would be the goal i think. Single kernel image can run as a 
> KVM host, as a KVM/HVM guest [if CPU support] or as a KVM/LL guest [if 
> no CPU support]. There's no hypercall overhead on native kernels, we 
> have binary-patching infrastructure in place to turn them into NOPs. 
> (which makes it quite close to zero-cost)
>
> right now the KVM paravirtualization work we are doing is gradually 
> transitioning KVM/HVM towards KVM/LL in essence, by eliminating all VM 
> exit reasons from KVM/HVM and turning them into hypercalls. Once that 
> has been achieved, KVM/LL could be implemented by an extra ll.c module 
> in drivers/kvm/, which does the cr3 tricks and pagetable maintainance 
> and CPU state save/restore, fault/trap/irq passback (and not much else).
>
> So i see very nice short and long term synergy between native Linux, 
> KVM/HVM guests and KVM/LL guests. What is an hypercall-accelerated 
> driver under KVM/HVM is a paravirtual driver on KVM/LL. What is a 
> hypercall-based speedup for KVM/HVM is a paravirtual facility for 
> KVM/LL. One and the same thing serves both purposes.
>
>   

If you have a CONFIG_PARAVIRT guest, I believe it will always be faster 
to run it without hardware assisted virtualization:

- you cannot eliminate vmexits due to host interrupts
- a hypercall will (probably) keep being more expensive than a syscall; 
it simply has a lot more work to do
- cr3 switches for CONFIG_PARAVIRT syscalls (which are necessary on 
x86_64) will probably become very cheap with tagged tlbs

So, in my opinion, CONFIG_PARAVIRT on non-hvm will eventually win on 
pure performance.  Hardware assisted virtualization will be used where 
you wish to run unmodified guests, and where ABI stability is critical.

I won't mind being proved wrong, though.  And of course we should share 
drivers and APIs between the two technologies.

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


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