Re: [Xen-devel] [PATCH RFC v2 00/12] xen/x86: use per-vcpu stacks for 64 bit pv domains

[Juergen Gross <jgross@xxxxxxxx>]

On 23/01/18 17:45, George Dunlap wrote:
> On 01/22/2018 06:39 PM, Andrew Cooper wrote:
>> Juergen: you're now adding a LTR into the context switch path which
>> tends to be very slow.  I.e. As currently presented, this series
>> necessarily has a higher runtime overhead than Jan's XPTI.
> 
> So here are a repeat of the "hypervisor compile" tests I did, comparing
> the different XPTI-like series so far.
> 
> # Experimental setup:
> Host:
>  - Intel(R) Xeon(R) CPU E5630  @ 2.53GHz
>  - 4 pcpus
>  - Memory: 4GiB
> Guest:
>  - 4vcpus, 512MiB, blkback to raw file
>  - CentOS 6 userspace
>  - Linux 4.14 kernel with PV / PVH / PVHVM / KVM guest support (along
> with expected drivers) built-in
> Test:
>  - cd xen-4.10.0
>  - make -C xen clean
>  - time make -j 4 xen
> 

...

> * Staging + Juergen's v2 series
> real    1m3.018s
> user    2m52.217s
> sys     0m40.357s
> 
> Result: 63s (0% overhead)
> 
> Unfortunately, I can't really verify that Juergen's patches are having
> any effect; there's no printk indicating whether it's enabling the
> mitigation or not.  I have verified that the changeset reported in `xl
> dmesg` corresponds to the branch I have with the patches applied.
> 
> So it's *possible* something has gotten mixed up, and the mitigation
> isn't being applied; but if it *is* applied, the performance is
> significantly better than the "band-aid" XPTI.

As there is no real mitigation in place, but only the needed rework of
the interrupt handling and context switching, anything not next to
xpti=off would have been disappointing for me. :-)

I'll add some statistics in the next patches so it can be verified the
patches are really doing something.

Thanks for doing the tests,


Juergen

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