On 07/09/17 16:03, gengdongjiu wrote: >> On 07/09/17 12:49, gengdongjiu wrote: >>> >>> >>> On 2017/9/7 18:13, Marc Zyngier wrote: >>>> On 07/09/17 11:05, gengdongjiu wrote: >>>>> Hi James, >>>>> >>>>> On 2017/9/7 17:20, James Morse wrote: >>>>>> Hi Dongjiu Geng, >>>>>> >>>>>> On 07/09/17 06:54, Dongjiu Geng wrote: >>>>>>> In VHE mode, host kernel runs in the EL2 and can enable 'User >>>>>>> Access Override' when fs==KERNEL_DS so that it can access kernel >>>>>>> memory. However, PSTATE.UAO is set to 0 on an exception taken from >>>>>>> EL1 to EL2. Thus when VHE is used and exception taken from a guest >>>>>>> UAO will be disabled and host will use the incorrect PSTATE.UAO. >>>>>>> So check and reset the PSTATE.UAO when switching to host. >>>>>> >>>>>> This would only be a problem if KVM were calling into world-switch >>>>>> with fs==KERNEL_DS. I can't see where this happens. >>>>> Not only KVM, may also kernel sets the fs == KERNEL_DS before >>>>> calling into world-switch >>>> >>>> How? Please describe the exact sequence of event that lead to this >>>> situation with the current code base. >>> >>> Hi Marc, >>> >>> Different tasks have different fs, such as USER_DS or KERNEL_DS. In >>> the context switch, it will restore the task's fs. Thus, that depends >>> on task itself, as shown below code. UAO is different with PAN, PAN will be always enabled if hardware CPU supports PAN feature, but >> UAO is dynamical change. >> >> You haven't answered my question: There is exactly one point where we enter the world-switch. Show me that, at this point, PSTATE.UAO >> *before* the call is different from PSTATE.UAO after the call. Give me the exact sequence of event that leads to this situation. Show me a >> stack trace. > > Hi Marc, > > If using current mainline KVM code + Qemu and modify nothing, may not > exist broken issue, because the Qemu progress FS should be USER_DS. Thanks. You've just been wasting everybody's time for two days. In case you haven't noticed, mainline KVM code is the *ONLY* thing we care about. > But if I make a modification for user space( Qemu or KVM tools) to > change its FS property to KERNEL_DS or use third party application > with KERNEL_DS FS to run the guest, it will have problem(the > KERNEL_DS is cleared). If you think my case is reasonable and should > support, I can show you the calling stack trace. If you think, my > case is not reasonable and KVM should not support the application > with KERNEL_DS fs to run guest. You can ignore this patch, thanks. I really cannot think of a good reason why we'd want to do that. Playing with set_fs() is almost universally wrong, and I'm certainly going to oppose to any change in that area unless the code that calls set_fs() has been made public and properly reviewed. Until then, UAO/PAN will stay as they are unless you prove that our current code is wrong. M. -- Jazz is not dead. It just smells funny...
