Re: paravirtual drivers?

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

Tomasz Chmielewski wrote:
> Anthony Liguori schrieb:
>
> (...)
>
>> 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).
>
> How does one enable SCSI emulation?

Right now, you have to modify the source code :-/

> "qemu-system-x86_64 -h" gives no hints about SCSI (at least in 
> KVM-18); similarly, I didn't find anything in the archive or in the wiki.
>
>
> Certainly, there is a kvm-18/qemu/hw/scsi-disk.c file, so it's like 
> the feature is there...
>
> Is it possible to boot a guest from a "SCSI-emulated" device?

Not yet.  There is work being done on an option ROM to allow SCSI boot 
support though.

Regards,

Anthony Liguori


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