linux-kernel - Re: [PATCH v4 0/8] KVM: x86: hyperv: PV TLB flush for Windows guests

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Date:   Fri, 18 May 2018 14:42:32 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     kvm <kvm@...r.kernel.org>,
        "the arch\/x86 maintainers" <x86@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley \(EOSG\)" <Michael.H.Kelley@...rosoft.com>,
        Mohammed Gamal <mmorsy@...hat.com>,
        Cathy Avery <cavery@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 0/8] KVM: x86: hyperv: PV TLB flush for Windows guests

Wanpeng Li <kernellwp@...il.com> writes:

> 2018-05-18 19:19 GMT+08:00 Vitaly Kuznetsov <vkuznets@...hat.com>:
>> Vitaly Kuznetsov <vkuznets@...hat.com> writes:
>>
>>> Wanpeng Li <kernellwp@...il.com> writes:
>>>
>>>> Hi Vitaly,
>>>> 2018-05-16 23:21 GMT+08:00 Vitaly Kuznetsov <vkuznets@...hat.com>:
>>>>> Changes since v3 [Radim Krcmar]:
>>>>> - PATCH2 fixing 'HV_GENERIC_SET_SPARCE_4K' typo added.
>>>>> - PATCH5 introducing kvm_make_vcpus_request_mask() API added.
>>>>> - Fix undefined behavior for hv->vp_index >= 64.
>>>>> - Merge kvm_hv_flush_tlb() and kvm_hv_flush_tlb_ex()
>>>>> - For -ex case preload all banks with a single kvm_read_guest().
>>>>>
>>>>> Description:
>>>>>
>>>>> This is both a new feature and a bugfix.
>>>>>
>>>>> Bugfix description:
>>>>>
>>>>> It was found that Windows 2016 guests on KVM crash when they have > 64
>>>>> vCPUs, non-flat topology (>1 core/thread per socket; in case it has >64
>>>>> sockets Windows just ignores vCPUs above 64) and Hyper-V enlightenments
>>>>
>>>> We try the below command line, the Windows 2016 guest successfully to
>>>> login and there are 80 vCPUs can be observed in the guest w/o the
>>>> patchset, why you mentioned the crash and ignore?
>>>>
>>>> /usr/local/bin/qemu-system-x86_64 -machine  pc-i440fx-rhel7.3.0  -m
>>>> 8192 -smp 80,sockets=2,cores=40,threads=1  -device
>>>> ide-drive,bus=ide.0,drive=test -drive
>>>> id=test,if=none,file=/instanceimage/359b18ab-05bb-460d-9b53-89505bca68ed/359b18ab-05bb-460d-9b53-89505bca68ed_vda_1.qcow2
>>>>  -net nic,model=virtio -net user -monitor stdio -usb -usbdevice tablet
>>>> --enable-kvm --cpu host -vnc 0.0.0.0:2
>>>
>>> Crash happens when you manifest yourself as Hyper-V, you can do this by
>>> adding any 'hv-*' feature (e.g. try '-cpu host,hv_vpindex').
>>
>> Oh, and the 'ignore' happens when you pass more than 64 sockets
>> (somthing like "-smp 128,sockets=128,cores=1,threads=1") -- and this
>> happens regardless of Hyper-V enlightenments. But I guess it's just
>> because Windows doesn't support more than 64 sockets.
>
> Is there an option in the guest to avoid to check pvtlb support in hyperv?
>

You mean to tell Windows to not use PV TLB flush when it's available? I
have no idea. My guess would be that it's left up to the hypervisor: if
the feature is available Windows will use it.

-- 
  Vitaly