Re: [Qemu-devel] An emulation failure occurs, if I hotplug vcpus immediately after the VM start

[David Hildenbrand]

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.

> 
> One fix could be to add a KVM_SET_USER_MEMORY_REGIONS ioctl that
> replaces the entire memory map atomically.
> 
> Paolo
> 


-- 

Thanks,

David / dhildenb