From mboxrd@z Thu Jan 1 00:00:00 1970 From: Konrad Rzeszutek Wilk Subject: Re: PVHVM drivers in upstream linux kernel Date: Tue, 2 Dec 2014 10:11:01 -0500 Message-ID: <20141202151101.GG27869@laptop.dumpdata.com> References: <547D88EF.4050206@suse.com> <547D9A4A.6010201@citrix.com> <1417518314.24320.9.camel@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <1417518314.24320.9.camel@citrix.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Ian Campbell Cc: Juergen Gross , "xen-devel@lists.xensource.com" , David Vrabel List-Id: xen-devel@lists.xenproject.org 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. > > > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xen.org > http://lists.xen.org/xen-devel