On Thu, Nov 12, 2015 at 10:02:10AM +0530, Aravinda Prasad wrote: > > > On Thursday 12 November 2015 09:08 AM, David Gibson wrote: > > On Thu, Nov 12, 2015 at 01:24:19PM +1100, Daniel Axtens wrote: > >> Aravinda Prasad <aravi...@linux.vnet.ibm.com> writes: > >> > >>> This patch modifies KVM to cause a guest exit with > >>> KVM_EXIT_NMI instead of immediately delivering a 0x200 > >>> interrupt to guest upon machine check exception in > >>> guest address. Exiting the guest enables QEMU to build > >>> error log and deliver machine check exception to guest > >>> OS (either via guest OS registered machine check > >>> handler or via 0x200 guest OS interrupt vector). > >>> > >>> This approach simplifies the delivering of machine > >>> check exception to guest OS compared to the earlier approach > >>> of KVM directly invoking 0x200 guest interrupt vector. > >>> In the earlier approach QEMU patched the 0x200 interrupt > >>> vector during boot. The patched code at 0x200 issued a > >>> private hcall to pass the control to QEMU to build the > >>> error log. > >>> > >>> This design/approach is based on the feedback for the > >>> QEMU patches to handle machine check exception. Details > >>> of earlier approach of handling machine check exception > >>> in QEMU and related discussions can be found at: > >>> > >>> https://lists.nongnu.org/archive/html/qemu-devel/2014-11/msg00813.html > >> > >> I've poked at the MCE code, but not the KVM MCE code, so I may be > >> mistaken here, but I'm not clear on how this handles errors that the > >> guest can recover without terminating. > >> > >> For example, a Linux guest can handle a UE in guest userspace by killing > >> the guest process. A hypthetical non-linux guest with a microkernel > >> could even survive UEs in drivers. > >> > >> It sounds from your patch like you're changing this behaviour. Is this > >> right? > > > > So, IIUC. Once the qemu pieces are in place as well it shouldn't > > change this behaviour: KVM will exit to qemu, qemu will log the error > > information (new), then reinject the MC to the guest which can still > > handle it as you describe above. > > Yes. With KVM and QEMU both in place this will not change the behavior. > QEMU will inject the UE to guest and the guest handles the UE based on > where it occurred. For example if an UE happens in a guest process > address space, that process will be killed. > > > > > But, there could be a problem if you have a new kernel with an old > > qemu, in that case qemu might not understand the new exit type and > > treat it as a fatal error, even though the guest could actually cope > > with it. > > In case of new kernel and old QEMU, the guest terminates as old QEMU > does not understand the NMI exit reason. However, this is the case with > old kernel and old QEMU as they do not handle UE belonging to guest. The > difference is that the guest kernel terminates with different error > code.
Ok.. assuming the guest has code to handle the UE in 0x200, why would the guest terminate with old kernel and old qemu? I haven't quite followed the logic. > > old kernel and old QEMU -> guest panics [1] irrespective of where UE > happened in guest address space. > old kernel and new QEMU -> guest panics. same as above. > new kernel and old QEMU -> guest terminates with unhanded NMI error > irrespective of where UE happened in guest > new kernel and new QEMU -> guest handles UEs in process address space > by killing the process. guest terminates > for UEs in guest kernel address space. > > [1] https://lists.ozlabs.org/pipermail/linuxppc-dev/2014-June/118329.html > > > > > Aravinda, do we need to change this so that qemu has to explicitly > > enable the new NMI behaviour? Or have I missed something that will > > make that case work already. > > I think we don't need to explicitly enable the new behavior. With new > kernel and new QEMU this should just work. As mentioned above this is > already broken for old kernel/QEMU. Any thoughts? > > Regards, > Aravinda > > > > > > > > > _______________________________________________ > > Linuxppc-dev mailing list > > linuxppc-...@lists.ozlabs.org > > https://lists.ozlabs.org/listinfo/linuxppc-dev > > > -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson
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