Re: [PATCH 1/7] KVM: arm64: Update comment when skipping guest MMIO access instruction

[Will Deacon <will@kernel.org>]

On Sun, Jul 26, 2020 at 12:08:28PM +0100, Marc Zyngier wrote:
> On Fri, 24 Jul 2020 15:35:00 +0100,
> Will Deacon <will@kernel.org> wrote:
> > 
> > If a 32-bit guest accesses MMIO using a 16-bit Thumb-2 instruction that
> > is reported to the hypervisor without a valid syndrom (for example,
> > because of the addressing mode), then we may hand off the fault to
> > userspace. When resuming the guest, we unconditionally advance the PC
> > by 4 bytes, since ESR_EL2.IL is always 1 for data aborts generated without
> > a valid syndrome. This is a bit rubbish, but it's also difficult to see
> > how we can fix it without potentially introducing regressions in userspace
> > MMIO fault handling.
> 
> Not quite, see below.
> 
> > 
> > Update the comment when skipping a guest MMIO access instruction so that
> > this corner case is at least written down.
> > 
> > Cc: Marc Zyngier <maz@kernel.org>
> > Cc: Quentin Perret <qperret@google.com>
> > Signed-off-by: Will Deacon <will@kernel.org>
> > ---
> >  arch/arm64/kvm/mmio.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/arch/arm64/kvm/mmio.c b/arch/arm64/kvm/mmio.c
> > index 4e0366759726..b54ea5aa6c06 100644
> > --- a/arch/arm64/kvm/mmio.c
> > +++ b/arch/arm64/kvm/mmio.c
> > @@ -113,6 +113,13 @@ int kvm_handle_mmio_return(struct kvm_vcpu *vcpu, struct kvm_run *run)
> >  	/*
> >  	 * The MMIO instruction is emulated and should not be re-executed
> >  	 * in the guest.
> > +	 *
> > +	 * Note: If user space handled the emulation because the abort
> > +	 * symdrome information was not valid (ISV set in the ESR), then
> 
> nits: syndrome, ISV *clear*.

Duh, thanks.

> > +	 * this will assume that the faulting instruction was 32-bit.
> > +	 * If the faulting instruction was a 16-bit Thumb instruction,
> > +	 * then userspace would need to rewind the PC by 2 bytes prior to
> > +	 * resuming the vCPU (yuck!).
> >  	 */
> >  	kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));
> >  
> 
> That's not how I read it. On ESR_EL2.ISV being clear, and in the
> absence of KVM_CAP_ARM_NISV_TO_USER being set, we return a -ENOSYS from
> io_mem_abort(), exiting back to userspace *without* advertising a MMIO
> access. The VMM is free to do whatever it can to handle it (i.e. not
> much), but crucially we don't go via kvm_handle_mmio_return() on
> resuming the vcpu (unless the VMM sets run->exit_reason to
> KVM_EXIT_MMIO, but that's clearly its own decision).
> 
> Instead, the expectation is that userspace willing to handle an exit
> resulting in ESR_EL2.ISV being clear would instead request a
> KVM_EXIT_ARM_NISV exit type (by enabling KVM_CAP_ARM_NISV_TO_USER),
> getting extra information in the process such as as the fault
> IPA). KVM_EXIT_ARM_NISV clearly states in the documentation:
> 
>   "Note that KVM does not skip the faulting instruction as it does for
>    KVM_EXIT_MMIO, but userspace has to emulate any change to the
>    processing state if it decides to decode and emulate the instruction."

Thanks, I think you're right. I _thought_ we always reported EXIT_MMIO
for write faults on read-only memslots (as per the documented behaviour),
but actually that goes down the io_mem_abort() path too and so the
skipping only ever occurs when the syndrome is valid.

I'll drop this patch.

Will

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