On Wed, Feb 14, 2024 at 12:55:11PM -0800, Oliver Upton wrote: > On Wed, Feb 14, 2024 at 08:09:52PM +0000, Marc Zyngier wrote: > > > If the order of restore from userspace is CBASER, CWRITER, CREADR then > > > we **wind up replaying the entire command queue**. While insane, I'm > > > pretty sure it is legal for the guest to write garbage after the read > > > pointer has moved past a particular command index. > > > > > > Fsck!!! > > > > This is documented Documentation/virt/kvm/devices/arm-vgic-its.rst to > > some extent, and it is allowed for the guest to crap itself on behalf > > of userspace if the ordering isn't respected. > > Ah, fair, I missed the documentation here. If we require userspace to > write CTLR last then we _should_ be fine, but damn is this a tricky set > of expectations. > > > > So, how about we do this: > > > > > > - Provide a uaccess hook for CWRITER that changes the write-pointer > > > without processing any commands > > > > > > - Assert an invariant that at any time CWRITER or CREADR are read from > > > userspace that CREADR == CWRITER. Fail the ioctl and scream if that > > > isn't the case, so that way we never need to worry about processing > > > 'in-flight' commands at the destination. > > > > Are we guaranteed that we cannot ever see CWRITER != CREADR at VM > > dumping time? I'm not convinced that we cannot preempt the vcpu thread > > at the right spot, specially given that you can have an arbitrary > > large batch of commands to execute. > > > > Just add a page-fault to the mix, and a signal pending. Pronto, you > > see a guest exit and you should be able to start dumping things > > without the ITS having processed much. I haven't tried, but that > > doesn't seem totally unlikely. > > Well, we would need to run all userspace reads and writes through the > cmd_lock in this case, which is what we already do for the CREADR > uaccess hook. To me the 'racy' queue accessors only make sense for guest > accesses, since the driver is expecting to poll for completion in that > case. My proposed invariant cannot be maintained, of course, since userspace can do whatever it pleases on the cmdq pointers. > Otherwise we decide the existing rules for restoring the ITS are fine > and I get to keep my funky driver :) > > -- > Thanks, > Oliver >
