Avi Kivity <avi.kivity@xxxxxxxxx> wrote: > On 03/09/2015 07:51 PM, Nadav Amit wrote: >> Avi Kivity <avi.kivity@xxxxxxxxx> wrote: >> >>> On 03/03/2015 11:52 AM, Paolo Bonzini wrote: >>>>> In this >>>>> case, the VM might expect exceptions when PTE bits which are higher than the >>>>> maximum (reported) address width are set, and it would not get such >>>>> exceptions. This problem can easily be experienced by small change to the >>>>> existing KVM unit-tests. >>>>> >>>>> There are many variants to this problem, and the only solution which I >>>>> consider complete is to report to the VM the maximum (52) physical address >>>>> width to the VM, configure the VM to exit on #PF with reserved-bit >>>>> error-codes, and then emulate these faulting instructions. >>>> Not even that would be a definitive solution. If the guest tries to map >>>> RAM (e.g. a PCI BAR that is backed by RAM) above the host MAXPHYADDR, >>>> you would get EPT misconfiguration vmexits. >>>> >>>> I think there is no way to emulate physical address width correctly, >>>> except by disabling EPT. >>> Is the issue emulating a higher MAXPHYADDR on the guest than is available >>> on the host? I don't think there's any need to support that. >>> >>> Emulating a lower setting on the guest than is available on the host is, I >>> think, desirable. Whether it would work depends on the relative priority >>> of EPT misconfiguration exits vs. page table permission faults. >> Thanks for the feedback. >> >> Guest page-table permissions faults got priority over EPT misconfiguration. >> KVM can even be set to trap page-table permission faults, at least in VT-x. >> Anyhow, I don’t think it is enough. > > Why is it not enough? If you trap a permission fault, you can inject any exception error code you like. Because there is no real permission fault. In the following example, the VM expects one (VM’s MAXPHYADDR=40), but there isn’t (Host’s MAXPHYADDR=46), so the hypervisor cannot trap it. It can only trap all #PF, which is obviously too intrusive. >> Here is an example >> >> My machine has MAXPHYADDR of 46. I modified kvm-unit-tests access test to >> set pte.45 instead of pte.51, which from the VM point-of-view should cause >> the #PF error-code indicate the reserved bits are set (just as pte.51 does). >> Here is one error from the log: >> >> test pte.p pte.45 pde.p user: FAIL: error code 5 expected d >> Dump mapping: address: 123400000000 >> ------L4: 304b007 >> ------L3: 304c007 >> ------L2: 304d001 >> ------L1: 200002000001 > > This is with an ept misconfig programmed into that address, yes? A reserved bit in the PTE is set - from the VM point-of-view. If there wasn’t another cause for #PF, it would lead to EPT violation/misconfig. >> As you can see, the #PF should have had two reasons: reserved bits, and user >> access to supervisor only page. The error-code however does not indicate the >> reserved-bits are set. >> >> Note that KVM did not trap any exit on that faulting instruction, as >> otherwise it would try to emulate the instruction and assuming it is >> supported (and that the #PF was not on an instruction fetch), should be able >> to emulate the #PF correctly. >> [ The test actually crashes soon after this error due to these reasons. ] >> >> Anyhow, that is the reason for me to assume that having the maximum >> MAXPHYADDR is better. > > Well, that doesn't work for the reasons Paolo noted. The guest can have a ivshmem device attached, and map it above a host-supported virtual address, and suddenly it goes slow. I fully understand. That’s the reason I don’t have a reasonable solution. Regards, Nadav-- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
