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

[Andrew Cooper <andrew.cooper3@citrix.com>]

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