Hi Christoffer, On 03/11/17 12:45, Christoffer Dall wrote: > On Thu, Nov 02, 2017 at 12:14:28PM +0000, James Morse wrote: >> On 30/10/17 07:40, Christoffer Dall wrote: >>> On Thu, Oct 19, 2017 at 03:57:55PM +0100, James Morse wrote: >>>> Non-VHE systems take an exception to EL2 in order to world-switch into the >>>> guest. When returning from the guest KVM implicitly restores the DAIF >>>> flags when it returns to the kernel at EL1. >>>> >>>> With VHE none of this exception-level jumping happens, so KVMs >>>> world-switch code is exposed to the host kernel's DAIF values, and KVM >>>> spills the guest-exit DAIF values back into the host kernel. >>>> On entry to a guest we have Debug and SError exceptions unmasked, KVM >>>> has switched VBAR but isn't prepared to handle these. On guest exit >>>> Debug exceptions are left disabled once we return to the host and will >>>> stay this way until we enter user space. >>>> Give me a kick if you want this reworked as a fix (which will then >>>> conflict with this series), or a backportable version. >>> >>> I don't know of any real-world issues where some more graceful handling >>> of SErrors would make sense on older kernels, so I'm fine with just >>> merging this together with this series. >> >> What about debug? > Are we unmasking debug exceptions as we should with this patch? With this patch, yes, it directly restores the DAIF flags the arch code wants for irq-masked process-context. Debug is re-enabled. > If so, I suppose that could be required for something like kgdb or when > running KVM as a guest hypervisor (nested). > > In that case, we should probably provide a backport for stable, if we > think people are going to be running older kernels on VHE systems, which > they probably are. Okay, I will produce a backport once this gets merged. >>> On guest exit Debug exceptions are left disabled once we return to the host >>> and will stay this way until we enter user space. > [The indentation seems to indicate I wrote this, but I don't think I > did. I'm confused.] I quoted it from the commit message, but evidently not from this depth-of-reply. Sorry for the confusion. >> Today VHE:KVM causes the kernel to run with SError unmasked and debug disabled >> until the next return to user-space, whereas previously the kernel expected >> SError to be masked and debug enabled. >> >> >> (Reposting just the SError rework without this patch changes the kernel to >> expect SError to be unmasked, which isn't making this any worse.) > I'm sorry, I don't understand this discussion. What is today, and what English has failed me. I'll try again: v4.14-rc7 with VHE causes the kernel to run after guest-exit with SError unmasked and debug disabled until the next return to user-space. The arch code expects SError masked and debug enabled. In your kgdb example, if we switch-to a new task instead of returning to user space, it won't hit any break/watchpoints. > is previously, and are you suggesting we drop this patch, or that the > rest of this series is somehow going to be applied without this patch? I reposted just the SError rework, patches 1-10 without this patch. If merged, this would change the arch code to expect SError to be unmasked from process context, leaving just the debug disabled after VHE guest-exit. I was (hurriedly) trying to work out if reposting the SError-rework without this patch made the situation worse. Sorry for the confusion! James > Reset: I think this patch is fine in the context of this series.. I now > have no idea what we need to do in terms of older kernels. > > Thanks, > -Christoffer > _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
