Re: [Xen-devel] Re: Next steps with pv_ops for Xen

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> I am not quite clear about the purpose of pv-ops , what do we want to
> deal with by developping "pv-ops"? is it used for HVM or for PV or KVM
> or something ? I have seen it for a few months in the list ,and
> "pv-ops"is an active project ,but i am not clear about what is the aim
> of "pv-ops" ,could you give me an explanation about it

PV-ops is an API within Linux which is used to support paravirtualisation.

paravirt-ops makes it possible to compile a Linux kernel which can boot on 
bare hardware, or on Xen, or using VMI (VMware's paravirtualised interface), 
lguest, or any other VMM that is supported.  The resulting kernel can then 
boot on any of those and make proper use of paravirtualisation.

For instance, with 2.6.23 from kernel.org you should be able to compile a 
kernel that will boot both on bare hardware and in a Xen domU in PV mode.  
Various tricks are used to ensure that it will run with good performance on 
both.

pv-ops mostly deals with the paravirtualisation of the CPU.  IO devices such 
as block and network are handled using Xen-aware drivers rather similar to 
those in the XenSource Linux kernels, they are not part of pv-ops.

Cheers,
Mark


> Thanks in advance
>
> Mark Williamson åé:
> >> Hi Mark,
> >>
> >>> Maybe a change to the gntdev userspace API to allow batching
> >>> of mapping requests?
> >>
> >> Something along the lines of the following?
> >
> > Just like that :-D
> >
> > When you said "multiple syscalls per mapping" I assumed you meant that
> > we'd lose the batching you get by doing a mulicall.  If it's just a
> > couple of syscalls (plus, presumably a couple of hypercalls) per batch of
> > mappings, my gut says it's probably not going to hurt block performance. 
> > My guts have been wrong in (many!) ways before of course...
> >
> > I guess the overhead *could* be reduced even more by just having a magic
> > ioctl that did all the mmap-ing stuff in one operation, but that'd
> > probably be really gross if it wasn't necessary!  And I doubt it'd make
> > upstream very happy...
> >
> > We'll also be eliminating the overheads involved in having a blktap ring
> > for talking to userspace and having to move requests between that ring
> > and the real block ring, so there's some definite wins in overheads as
> > well.
> >
> > Cheers,
> > Mark



-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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