Re: kvm [2087]: load/store instruction decoding not implemented

["Richard W.M. Jones" <rjones@redhat.com>]

On Tue, Feb 24, 2015 at 09:15:18PM +0900, Peter Maydell wrote:
> On 24 February 2015 at 20:59, Richard W.M. Jones <rjones@redhat.com> wrote:
> >
> > https://bugzilla.redhat.com/show_bug.cgi?id=1194366
> >
> > Has anyone seen this KVM error?  Or have suggestions how to debug it
> > further?
> >
> >   kvm [2028]: load/store instruction decoding not implemented
> 
> This is a fairly common thing to run into and google is bound
> to have references to past discussions. What has happened here
> is that the guest has attempted a "complex" load/store instruction
> to an area of RAM which is not mapped (ie not guest RAM).
> For this class of instructions the hardware doesn't provide
> syndrome information to allow us to figure out the address/size
> etc of the access, so we would have to actually decode the
> offending instruction and emulate it; this emulation isn't
> implemented.
> 
> Complex insns are things like load-multiple (there's a complete
> list in the ARM ARM somewhere). Generally this indicates a guest
> bug because you really shouldn't be accessing devices with
> weird instructions like that (and you shouldn't be accessing
> unmapped memory at all).

I'm not super-familiar with the aarch64 instruction set, but
according to qemu the instruction is:

b8004403        str     w3, [x0],#4

(in __copy_to_user).  My interpretation is this is storing the
lower 32 bits of x3 into the storage pointed to by x0 (+ 4 bytes?)
Is that one of the complicated ones?

> At some point we might actually implement the decoding,
> which will probably just mean your guest crashes inside
> the VM rather than outside it.
> 
> > Qemu prints this before crashing:
> >
> >   error: kvm run failed Function not implemented
> >   (followed by a register dump)
> 
> That's not a QEMU crash, it's QEMU exiting noisily. You can
> use the register dump info in combination with the kernel
> address map to find out exactly what was trying the access
> that failed.
> 
> (Maybe we should add a line to that dump saying "this is not
> a QEMU crash" because it's kinda misleading :-))

Rich.

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