Re: [PATCH v2 1/2] KVM: s390: load guest access registers in MEM_OP ioctl

[Eric Farman <farman@linux.ibm.com>]

On Fri, 2024-02-16 at 10:40 +0100, Heiko Carstens wrote:
> On Thu, Feb 15, 2024 at 09:53:43PM +0100, Eric Farman wrote:
> > The routine ar_translation() can be reached by both the instruction
> > intercept path (where the access registers had been loaded with the
> > guest register contents), and the MEM_OP ioctls (which hadn't).
> > This latter case means that any ALET the guest expects to be used
> > would be ignored.
> > 
> > Fix this by swapping the host/guest access registers around the
> > MEM_OP ioctl, in the same way that the KVM_RUN ioctl does with
> > sync_regs()/store_regs(). The full register swap isn't needed here,
> > since only the access registers are used in this interface.
> > 
> > Introduce a boolean in the kvm_vcpu_arch struct to indicate the
> > guest ARs have been loaded into the registers. This permits a
> > warning to be emitted if entering this path without a proper
> > register setup.
> > 
> > Suggested-by: Christian Borntraeger <borntraeger@linux.ibm.com>
> > Signed-off-by: Eric Farman <farman@linux.ibm.com>
> > ---
> >  arch/s390/include/asm/kvm_host.h |  1 +
> >  arch/s390/kvm/gaccess.c          |  2 ++
> >  arch/s390/kvm/kvm-s390.c         | 11 +++++++++++
> >  3 files changed, 14 insertions(+)
> ...
> > diff --git a/arch/s390/kvm/gaccess.c b/arch/s390/kvm/gaccess.c
> > index 5bfcc50c1a68..33587bb4c9e8 100644
> > --- a/arch/s390/kvm/gaccess.c
> > +++ b/arch/s390/kvm/gaccess.c
> > @@ -391,6 +391,8 @@ static int ar_translation(struct kvm_vcpu
> > *vcpu, union asce *asce, u8 ar,
> >  	if (ar >= NUM_ACRS)
> >  		return -EINVAL;
> >  
> > +	WARN_ON_ONCE(!vcpu->arch.acrs_loaded);
> > +
> >  	save_access_regs(vcpu->run->s.regs.acrs);
> 
> Why not simply:
> 
> 	if (vcpu->arch.acrs_loaded)
> 		save_access_regs(vcpu->run->s.regs.acrs);
> 
> ?
> 
> This will always work, and the WARN_ON_ONCE() would not be needed.
> Besides
> that: _if_ the WARN_ON_ONCE() would trigger, damage would have
> happened
> already: host registers would have been made visible to the guest.
> 
> Or did I miss anything?

You're right that the suggestion to skip the save_access_regs() call in
this way would get the ALET out of the guest correctly, but the actual
CPU AR hadn't yet been loaded with the guest contents. Thus, the data
copy would be done with the host access register rather than the
guest's, which is why I needed to add those two extra hunks to do an AR
swap around the MEM_OP interface. Without that, the selftest in patch 2
continues to fail.

If the WARN triggers, damage will be done if the ARs get copied back to
the vcpu->run space (I don't believe any damage has occurred at the
time of the WARN). That's what's happening today and I'd like to
address, but there's no indication of what's happened. Perhaps I need
to combine the two ideas? Do the WARN, but remove the
save_access_regs() call since it gets done again once the registers are
swapped back. Or keep it, and dig out the RFC code that stores the
current ARs into a temporary variable instead?

Thanks,
Eric