> -----Original Message----- > From: David Hildenbrand [mailto:david@xxxxxxxxxx] > 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
