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Re: [Qemu-devel] An emulation failure occurs, if I hotplug vcpus immedia
From: |
David Hildenbrand |
Subject: |
Re: [Qemu-devel] An emulation failure occurs, if I hotplug vcpus immediately after the VM start |
Date: |
Thu, 7 Jun 2018 13:43:49 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 |
On 07.06.2018 13:13, Gonglei (Arei) wrote:
>
>> -----Original Message-----
>> From: David Hildenbrand [mailto:address@hidden
>> Sent: Thursday, June 07, 2018 6:40 PM
>> Subject: Re: An emulation failure occurs,if I hotplug vcpus immediately
>> after the
>> VM start
>>
>> On 06.06.2018 15:57, Paolo Bonzini wrote:
>>> On 06/06/2018 15:28, Gonglei (Arei) wrote:
>>>> gonglei********: mem.slot: 3, mem.guest_phys_addr=0xc0000,
>>>> mem.userspace_addr=0x7fc343ec0000, mem.flags=0, memory_size=0x0
>>>> gonglei********: mem.slot: 3, mem.guest_phys_addr=0xc0000,
>>>> mem.userspace_addr=0x7fc343ec0000, mem.flags=0,
>> memory_size=0x9000
>>>>
>>>> When the memory region is cleared, the KVM will tell the slot to be
>>>> invalid (which it is set to KVM_MEMSLOT_INVALID).
>>>>
>>>> If SeaBIOS accesses this memory and cause page fault, it will find an
>>>> invalid value according to gfn (by __gfn_to_pfn_memslot), and finally
>>>> it will return an invalid value, and finally it will return a
>>>> failure.
>>>>
>>>> So, My questions are:
>>>>
>>>> 1) Why don't we hold kvm->slots_lock during page fault processing?
>>>
>>> Because it's protected by SRCU. We don't need kvm->slots_lock on the
>>> read side.
>>>
>>>> 2) How do we assure that vcpus will not access the corresponding
>>>> region when deleting an memory slot?
>>>
>>> We don't. It's generally a guest bug if they do, but the problem here
>>> is that QEMU is splitting a memory region in two parts and that is not
>>> atomic.
>>
>> BTW, one ugly (but QEMU-only) fix would be to temporarily pause all
>> VCPUs, do the change and then unpause all VCPUs.
>>
>
> The updating process of memory region is triggered by vcpu thread, not
> the main process though.
Yes, I also already ran into this problem. Because it involves calling
pause_all_vcpus() from a VCPU thread. I sent a patch for that already,
but we were able to solve the s390x problem differently.
https://patchwork.kernel.org/patch/10331305/
The major problem of pause_all_vcpus() is that it will temporarily drop
the iothread mutex, which can result in "funny" side effects :) Handling
parallel call to pause_all_vcpus() is the smaller issue.
So right now, it can only be used from the main thread.
>
> Thanks,
> -Gonglei
>
>>>
>>> One fix could be to add a KVM_SET_USER_MEMORY_REGIONS ioctl that
>>> replaces the entire memory map atomically.
>>>
>>> Paolo
>>>
>>
>>
>> --
>>
>> Thanks,
>>
>> David / dhildenb
--
Thanks,
David / dhildenb
- Re: [Qemu-devel] An emulation failure occurs, if I hotplug vcpus immediately after the VM start, (continued)
- Re: [Qemu-devel] An emulation failure occurs, if I hotplug vcpus immediately after the VM start, David Hildenbrand, 2018/06/07
- Re: [Qemu-devel] An emulation failure occurs, if I hotplug vcpus immediately after the VM start, Paolo Bonzini, 2018/06/07
- Re: [Qemu-devel] An emulation failure occurs, if I hotplug vcpus immediately after the VM start, David Hildenbrand, 2018/06/07
- Re: [Qemu-devel] An emulation failure occurs, if I hotplug vcpus immediately after the VM start, 浙大邮箱, 2018/06/07
- Re: [Qemu-devel] An emulation failure occurs, if I hotplug vcpus immediately after the VM start, David Hildenbrand, 2018/06/11
- Re: [Qemu-devel] An emulation failure occurs, if I hotplug vcpus immediately after the VM start, Gonglei (Arei), 2018/06/11
- Re: [Qemu-devel] An emulation failure occurs, if I hotplug vcpus immediately after the VM start, David Hildenbrand, 2018/06/11
- Re: [Qemu-devel] An emulation failure occurs, if I hotplug vcpus immediately after the VM start, Gonglei (Arei), 2018/06/11
- Re: [Qemu-devel] An emulation failure occurs, if I hotplug vcpus immediately after the VM start, David Hildenbrand, 2018/06/07
- Re: [Qemu-devel] An emulation failure occurs, if I hotplug vcpus immediately after the VM start, Gonglei (Arei), 2018/06/07
- Re: [Qemu-devel] An emulation failure occurs, if I hotplug vcpus immediately after the VM start,
David Hildenbrand <=