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

[Paolo Bonzini]

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.

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

Paolo