On Thu, 5 Sep 2019 at 09:25, Christoffer Dall <christoffer.dall@xxxxxxx> wrote: > > On Thu, Sep 05, 2019 at 09:16:54AM +0100, Peter Maydell wrote: > > This is true, but the problem is that barfing out to userspace > > makes it harder to debug the guest because it means that > > the VM is immediately destroyed, whereas AIUI if we > > inject some kind of exception then (assuming you're set up > > to do kernel-debug via gdbstub) you can actually examine > > the offending guest code with a debugger because at least > > your VM is still around to inspect... > > > > Is it really going to be easier to debug a guest that sees behavior > which may not be architecturally correct? For example, seeing a data > abort on an access to an MMIO region because the guest used a strange > instruction? Yeah, a data abort is not ideal. You could UNDEF the insn, which probably is more likely to result in getting control in the debugger I suppose. As for whether it's going to be easier to debug, for the user who reported this in the first place it certainly was. (Consider even a simple Linux guest not under a debugger -- if we UNDEF the insn the guest kernel will print a helpful backtrace so you can tell where the problem is; at the moment we just print a register dump from the host kernel, which is a lot less informative.) > I appreaciate that the current way we handle this is confusing and has > led many people down a rabbit hole, so we should do better. > > Would a better approach not be to return to userspace saying, "we can't > handle this in the kernel, you decide", without printing the dubious > kernel error message. Printing the message in the kernel is the best clue we give the user at the moment that they've run into this problem; I would be wary of removing it (even if we decide to also do something else). > Then user space could suspend the VM and print a > lenghty explanation of all the possible problems there could be, or > re-inject something back into the guest, or whatever, for a particular > environment. In theory I guess so. In practice that's not what userspace currently in the wild does, and injecting an exception from userspace is a bit awkward (I dunno if kvmtool does it, QEMU only needs to in really obscure circumstances and was buggy in how it tried to do it until very recently)... thanks -- PMM _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
