Re: paravirtual drivers?

[Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>]

Tomasz Chmielewski wrote:
> As I understand, fully virtualized KVM guests (on Intel VT or AMD-V) can 
> perform with near native speeds only when it comes to the CPU - because 
> the things like network or block device are basically emulated.
>
>
> Among many changes, KVM in 2.6.21 has "Initial paravirtualization 
> support, which has much faster performance", and is based on KVM-15.
>
>
> At least two PV drivers are needed:
> - network
> - block device
>   

I don't believe a PV block driver is actually necessary.  Disk devices 
are actually quite slow (unless you have a large array).  With a few 
small changes (bump MMU cache up to 1024 and actually enable the aio 
subsystem), SCSI emulation does as well as Xen's PV block driver (about 
210 mb/sec whereas native is around 250 mb/sec).  There's a lot of room 
to optimize the SCSI emulation too since there are a number of copies in 
that path.

The SCSI emulation is a lot like what one would design in a PV driver.  
It supports multiple outstanding requests and doesn't require a whole 
lot of exits to submit a request (it uses scripts).

Regards,

Anthony Liguori

>
> What is the state of these PV drivers now? Is it usable on Linux now?
>
> Are Windows PV drivers planned, too? If yes, who is going to take the 
> lead here (Qumranet, community, not decided yet etc.?).
>
>
>
>   


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