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

[Gonglei (Arei)]

> -----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.

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