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

[George Dunlap <george.dunlap@citrix.com>]

On 01/23/2018 04:56 PM, Juergen Gross wrote:
> 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.

Well at very least there should be something in the boot scrool that
says, "Enabling Xen Pagetable protection (XPTI) for PV guests" or
something.  (That goes for the current round of XPTI as well really.)

 -George

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