On 23/10/20 18:59, Jim Mattson wrote: >> The problem is that page fault error code bits cannot be reconstructed >> from bits 0..2 of the EPT violation exit qualification, if bit 8 is >> clear in the exit qualification (that is, if the access causing the EPT >> violation is to a paging-structure entry). In that case bits 0..2 refer >> to the paging-structure access rather than to the final access. In fact >> advanced information is not available at all for paging-structure access >> EPT violations. > > True, but the in-kernel emulator can only handle a very small subset > of the available instructions. > > If bit 8 is set in the exit qualification, we should use the advanced > VM-exit information. If it's clear, we should just do a software page > walk of the guest's x86 page tables. The information that we need is _not_ that provided by the advanced VM-exit information (or by a page walk). If a page is neither writable nor executable, the advanced information doesn't say if the injected #PF should be a W=1 or a F=1 fault. We need the information in bits 0..2 of the exit qualification for the final access, which however is not available for the paging-structure access. If bit 8 is set, however, we need not use the emulator indeed, as the W/F/U bits are all available from either the exit qualification or in the SS access rights. The access.flat test in kvm-unit-tests covers all this, so it will be easy to check the theory. Paolo > The in-kernel emulator should > only be used as a last resort on hardware that doesn't support the > advanced VM-exit information for EPT violations. >
