Hi kvm developers. I am having the strange problem that can only be reproduced on a core2duo CPU but not AMD FX or Intel Core I7. My code has 2 ways of setting the guest registers: one is the guest's ring0 stub that just pops all regs from stack and does iret to ring3. That works fine. But sometimes I use KVM_SET_SREGS and resume the VM directly to ring3. That randomly results in either a good run or invalid guest state return, or a page fault in guest. I tried to analyze when either of the above happens exactly, and I have a very strong suspection that the problem is in a way I update LDT. LDT is shared between guest and host with KVM_SET_USER_MEMORY_REGION, and I modify it on host. So it seems like if I just allocated the new LDT entry, there is a risk of invalid guest state, as if the guest's LDT still doesn't have it. If I modified some LDT entry, there can be a page fault in guest, as if the entry is still old. I've found that the one needs to check KVM_CAP_SYNC_MMU to safely write to the guest memory, but it doesn't seem to be documented well. Of course maybe my problem has nothing to do with that, but I think it does. So can it be that even though I check for the KVM_CAP_SYNC_MMU, writing to the guest memory from host is still unsafe? What is this KVM_CAP_SYNC_MMU actually all about?
