On Fri, Sep 06, 2019 at 02:31:42PM +0100, Peter Maydell wrote:
> On Fri, 6 Sep 2019 at 14:13, Christoffer Dall <christoffer.dall@xxxxxxx> wrote:
> > I'd prefer leaving it to userspace to worry about, but I thought Peter
> > said that had been problematic historically, which I took at face value,
> > but I could have misunderstood.
> >
> > If QEMU, kvmtool, and whatever the crazy^H cool kids are using in
> > userspace these days are happy emulating the exception, then that's a
> > viable approach. The main concern I have with that is whether they'll
> > all get it right, and since we already have the code in the kernel to do
> > this, it might make sense to re-use the kernel logic for it.
>
> Well, for QEMU we've had code that in theory might do this but
> in practice it's never been tested. Essentially the problem is
> that nobody ever wants to inject an exception from userspace
> except in incredibly rare cases like "trying to use h/w breakpoints
> simultaneously inside the VM and also to debug the VM from outside"
> or "we're running on RAS hardware that just told us that the VM's
> RAM was faulty". There's no even vaguely commonly-used usecase
> for it today; and this ABI suggestion adds another "this is in
> practice almost never going to happen" case to the pile. The
> codepath is unreliable in QEMU because (a) it requires getting
> syncing of sysreg values to and from the kernel right -- this is
> about the only situation where userspace wants to modify sysregs
> during execution of the VM, as opposed to just reading them -- and
> (b) we try to reuse the code we already have that does TCG exception
> injection, which might or might not be a design mistake, and
> (c) as noted above it's a never-actually-used untested codepath...
>
> So I think if I were you I wouldn't commit to the kernel ABI until
> somebody had at least written some RFC-quality patches for QEMU and
> tested that they work and the ABI is OK in that sense. (For the
> avoidance of doubt, I'm not volunteering to do that myself.)
> I don't object to the idea in principle, though.
>
> PS: the other "injecting exceptions to the guest" oddity is that
> if the kernel *does* find the ISV information and returns to userspace
> for it to handle the MMIO, there's no way for userspace to say
> "actually that address is supposed to generate a data abort".
>
That's a good point. A synchronous interface with a separate mechanism
to ask the kernel to inject an exception might be a good solution, if
there's an elegant way to do the latter. I'll have a look at that.
Thanks,
Christoffer
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