[PATCH v10 0/9] Hyper-V: paravirtualized remote TLB flushing and hypercall improvements

Wanpeng Li kernellwp at gmail.com
Mon Nov 6 09:57:04 UTC 2017

2017-11-06 17:14 GMT+08:00 Vitaly Kuznetsov <vkuznets at redhat.com>:
> Wanpeng Li <kernellwp at gmail.com> writes:
>> 2017-08-03 0:09 GMT+08:00 Vitaly Kuznetsov <vkuznets at redhat.com>:
>>> Changes since v9:
>>> - Rebase to 4.13-rc3.
>>> - Drop PATCH1 as it was already taken by Greg to char-misc tree. There're no
>>>   functional dependencies on this patch so the series can go through a different tree
>>>   (and it actually belongs to x86 if I got Ingo's comment right).
>>> - Add in missing void return type in PATCH1 [Colin King, Ingo Molnar, Greg KH]
>>> - A few minor fixes in what is now PATCH7: add pr_fmt, tiny style fix in
>>>   hyperv_flush_tlb_others() [Andy Shevchenko]
>>> - Fix "error: implicit declaration of function 'virt_to_phys'" in PATCH2
>>>   reported by kbuild test robot (#include <asm/io.h>)
>>> - Add Steven's 'Reviewed-by:' to PATCH9.
>>> Original description:
>>> Hyper-V supports hypercalls for doing local and remote TLB flushing and
>>> gives its guests hints when using hypercall is preferred. While doing
>>> hypercalls for local TLB flushes is probably not practical (and is not
>>> being suggested by modern Hyper-V versions) remote TLB flush with a
>>> hypercall brings significant improvement.
>>> To test the series I wrote a special 'TLB trasher': on a 16 vCPU guest I
>>> was creating 32 threads which were doing 100000 mmap/munmaps each on some
>>> big file. Here are the results:
>>> Before:
>>> # time ./pthread_mmap ./randfile
>>> real    3m33.118s
>>> user    0m3.698s
>>> sys     3m16.624s
>>> After:
>>> # time ./pthread_mmap ./randfile
>>> real    2m19.920s
>>> user    0m2.662s
>>> sys     2m9.948s
>>> This series brings a number of small improvements along the way: fast
>>> hypercall implementation and using it for event signaling, rep hypercalls
>>> implementation, hyperv tracing subsystem (which only traces the newly added
>>> remote TLB flush for now).
>> Hi Vitaly,
>> Could you attach your benchmark? I'm interested in to try the
>> implementation in paravirt kvm.
> Oh, this would be cool) I briefly discussed the idea with Radim (one of
> KVM maintainers) during the last KVM Forum and he wasn't opposed to the
> idea. Need to talk to Paolo too. Good thing is that we have everything

I talk with Paolo today and he points this feature to me, so I believe
he likes it. :) In addition,
I search Hypervisor Top Level Functional Specification v5.0b.pdf
document but didn't find a section introduce the Hyper-V:
paravirtualized remote TLB flushing and hypercall stuff, could you
point out?

Wanpeng Li

> in place for guests now (HAVE_RCU_TABLE_FREE is enabled globaly on x86).
> Please see the microbenchmark attached. Adjust defines in the beginning
> to match your needs. It is not anything smart, basically just a TLB
> trasher.
> In theory, the best result is achived when we're overcommiting the host
> by running multiple vCPUs on each pCPU. In this case PV tlb flush avoids
> touching vCPUs which are not scheduled and avoid the wait on the main
> CPU.
> --
>   Vitaly

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