Re: [RFC] Hypervisor, x86 emulation deprivileged

[Anthony PERARD <anthony.perard@citrix.com>]

On Tue, Jul 05, 2016 at 07:02:39AM -0600, Jan Beulich wrote:
> >>> On 05.07.16 at 13:22, <anthony.perard@citrix.com> wrote:
> > Hi,
> > 
> > I've taken over the work from Ben to have a deprivileged mode in the
> > hypervisor, but I'm unsure about which direction to take.
> > 
> > First, after understanding what have been done, and fixing a few things,
> > I did some benchmark to compare a simple "device" running in ring0 to
> > the same one running in ring3 and also in QEMU. This "device" would call
> > 'rdtsc' on 'outl' and return the value in 'inl' (I actually do not use
> > the value). The measurement is done from a kernel module in the guest
> > (simply rdtsc;inl;rdtsc multiple time). This is the result I've found:
> > 
> >     ring3 ~3.5x slower than ring0
> >     qemu   ~22x slower than ring0
> >           ~6.5x slower than ring3
> > 
> > So that would be the worst-case scenario, where an emulator barely do
> > anything.
> > 
> > 
> > There have been different methods proposed to do the depriv mode, in
> > <55A8D477.2060909@citrix.com>, one of which was to implement a per-vcpu
> > stack which could be more elegant.
> 
> Sadly my mail frontend doesn't let me search for message IDs (and
> this old a mail would have been purged anyway meanwhile), so I
> think (also considering how much time has passed) it would be better
> if you actually summarized where things stopped back then.

https://lists.xen.org/archives/html/xen-devel/2015-07/msg03507.html

It has been said that a per-vcpu stack would be too much work for a
short term project.

> > So, would you suggest that I start working on a per-vcpu stack? Or
> > should I continue with the current direction?
> 
> Was there any reason why using per-vCPU stacks would be assumed
> to meaningfully improve above numbers?

Probably not. I guess the context switch alone takes most time, and it
does not matter where the stack is and if there is a copy of it.

> I'm not sure pursuing this
> idea is really useful if more than a marginal performance degradation
> results.

Maybe the instruction emulator would be big enough that the impact of a
context swith would not matter as much? I don't know much about it so I
can not make a guess of how much code is running.

-- 
Anthony PERARD

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