Messages in this thread | | | Date | Fri, 20 Feb 2015 10:47:52 +0000 | From | Andrew Cooper <> | Subject | Re: [Xen-devel] NUMA_BALANCING and Xen PV guest regression in 3.20-rc0 |
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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.
> > 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.
> There was the whole odd crazy SHARED_KERNEL_PMD hackery too.
SHARED_KERNEL_PMD is an artefact of Xen living in the same virtual address space as a PV guest, and needing to maintain linear mappings.
~Andrew
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