Re: [PATCH 6/7] KVM: arm64: Handle stage-2 faults on stage-1 page-table walks earlier

[Will Deacon <will@kernel.org>]

On Sun, Jul 26, 2020 at 02:38:38PM +0100, Marc Zyngier wrote:
> On Fri, 24 Jul 2020 15:35:05 +0100,
> Will Deacon <will@kernel.org> wrote:
> > 
> > Stage-2 faults on stage-1 page-table walks can occur on both the I-side
> > and the D-side. It is IMPLEMENTATATION DEFINED whether they are reported
> > as reads or writes and, in the case that they are generated by an AT
> > instruction, they are reported with the CM bit set.
> > 
> > All of this deeply confuses the logic in kvm_handle_guest_abort();
> > userspace may or may not see the fault, depending on whether it occurs
> > on the data or the instruction side, and an AT instruction may be skipped
> > if the translation tables are held in a read-only memslot.
> 
> Yuk, that's indeed ugly. Well spotted. I guess the saving grace is
> that a S2 trap caused by an ATS1 instruction will be reported as
> S1PTW+CM, while the fault caused by a CMO is reported as *either*
> S1PTW *or* CM, but never both.

Hmm, is that right? If the translation faults at S2 for a CM instruction,
wouldn't it have both bits set?

> > Move the handling of stage-2 faults on stage-1 page-table walks earlier
> > so that they consistently result in either a data or an instruction abort
> > being re-injected back to the guest.
> 
> The instruction abort seems to be happening as the side effect of
> executing outside of a memslot, not really because of a S1PTW.

Not sure about that. If the instruction fetch generates an S2 abort during
translation, then we could be executing from within a memslot; it's the
location of the page-tables that matters.

However, I think that means things still aren't quite right with my patches
because we can end up calling io_mem_abort() from an instruction abort,
which won't have enough syndrome information to do anything useful. Hmm.

Stepping back, here's what I _think_ we want, although see the '(?)'
bits where I'm particularly unsure:


S2 instruction abort:
  * Not in memslot:		inject external iabt to guest
  * In R/O memslot:
    - S2 fault on S1 walk:	either EXIT_NISV or inject external iabt
				to guest (?)

S2 data abort:
  * Not in memslot:
    - S2 fault on S1 walk:	inject external dabt to guest
    - Cache maintenance:	skip instr
    - Syndrome valid		EXIT_MMIO
    - Syndrome invalid		EXIT_NISV
  * In R/O memslot:
    - S2 fault on S1 walk:	either EXIT_NISV or inject external dabt
				to guest (?)
    - Access is write (including cache maintenance (?)):
      - Syndrome valid		EXIT_MMIO
      - Syndrome invalid	EXIT_NISV


Everything else gets handled by handle_access_fault()/user_mem_abort().

What do you think?

> I wonder whether these S1PTW faults should be classified as external
> aborts instead (because putting your page tables outside of a memslot
> seems a bit bonkers).

I think that's what this patch does, since we end up in kvm_inject_dabt().

Will
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