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Re: [Xen-devel] PVHVM drivers in upstream linux kernel

On Tue, Dec 02, 2014 at 11:05:14AM +0000, Ian Campbell wrote:
> On Tue, 2014-12-02 at 10:54 +0000, David Vrabel wrote:
> > On 02/12/14 09:39, Juergen Gross wrote:
> > > Hi,
> > > 
> > > looking into the upstream linux sources I realized that the PVHVM
> > > drivers of XEN are only available with the pvops kernel. Is this on
> > > purpose? Shouldn't the frontend drivers, xen/platform-pci.c etc. be
> > > configurable without having to enable CONFIG_PARAVIRT?
> > 
> > I suppose that would be possible but I don't think it's a useful
> > configuration because you would lose PV spinlocks for example.
> IIRC the reason this hasn't been implemented until now is that
> refactoring would be required to the various bits of driver code which
> assumes PAE + PARAVIRT when they aren't strictly needed, e.g. grant
> table code. Whether its worth the churn at this stage is debatable, but
> I think the (in)ability to use PV spinlocks is a red-herring.
> Adding PV drivers to an HVM guest is a useful thing to do, even without
> PV spinlocks. PV IO gets you far more incremental benefit than the locks
> do, adding PV IO paths is the number 1 thing which should be done to any
> guest.
> One actual usecase is installing from a distro installer which isn't
> PAE, let alone PARAVIRT enabled[0], to get far enough that you can
> install a more capable PVHVM kernel with more bells and whistles.
> If there were distros around who refused wholesale to enable PARAVIRT
> even in a non-default kernel then it would be more likely that they
> could be convinced to enable a set of PV IO drivers, since they have 0
> impact on a non-PARAVIRT system, and still give significant benefit to
> Xen users. I don't know of any of the major distros are refusing
> PARAVIRT in this way though.
> Ian.
> [0] The default i386 Debian installer falls into this camp, but you can
> use the special PV Xen variant to install as PVHVM too so it's not so
> critical.

And the Fedora 21 LiveISO (32-bit) does too.
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