Re: [Xen-devel] NUMA_BALANCING and Xen PV guest regression in 3.20-rc0

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Fri, Feb 20, 2015 at 10:47:52AM +0000, Andrew Cooper wrote:
> On 20/02/15 01:49, Linus Torvalds wrote:
> > On Thu, Feb 19, 2015 at 5:05 PM, Kirill A. Shutemov
> > <kirill@shutemov.name> wrote:
> >> I'm feeling I miss very basic background on how Xen works, but why does it
> >> set _PAGE_GLOBAL on userspace entries? It sounds strange to me.
> > It is definitely strange. I'm guessing that it's some ancient Xen hack
> > for the early Intel virtualization that used to have absolutely
> > horrendous vmenter/exit costs, including very much the TLB overhead. \
> >
> > These days, Intel has address space identifiers, and doesn't flush the
> > whole TLB on VM entry/exit, so it's probably pointless to play games
> > with the global bit.
> 
> It was introduced in 2006, but has nothing to do with VT-x
> 
> http://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=6f562e72cdc4b7e1519e23be75f812aebbf41db3
> 
> As long mode drops segment limit checking, the only way to protect a
> 64bit PV kernel from its userspace (both of which run in ring3 on user
> pages) is to maintain two sets of pagetables and switch between them on
> guest kernel/user context switches.  The user set lack kernel mappings.
> 
> I can't comment about the performance impact of the patch (way before my
> time), but the justification was to try and reduce the overhead of guest
> context switches.

IIUC, it tries to reduce userspace->kernel switch in guest. It's still
hopeless: kernel mappings are always TLB-cold, right?

> > I get the feeling that a lot of Xen stuff is that kind of "legacy
> > hacks" that should just be cleaned up, but nobody has the energy or
> > the interest.
> 
> Time, mainly.
> 
> There certainly are areas which should be up for re-evaluation, given 9
> years of change in hardware.

Is Xen PV still widely used? I'm surprised that users can tolerate this
kind of overhead.

-- 
 Kirill A. Shutemov