[PATCH v3 7/9] KVM: arm/arm64: vgic: Permit uaccess writes to return errors

[christoffer.dall@arm.com (Christoffer Dall)]

On Mon, Jul 16, 2018 at 09:28:00AM +0100, Marc Zyngier wrote:
> On 14/07/18 18:05, Christoffer Dall wrote:
> > Currently we do not allow any vgic mmio write operations to fail, which
> > makes sense from mmio traps from the guest.  However, we should be able
> > to report failures to userspace, if userspace writes incompatible values
> > to read-only registers.  Rework the internal interface to allow errors
> > to be returned on the write side for userspace writes.
> >
> > Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
> > ---
> >  virt/kvm/arm/vgic/vgic-mmio-v2.c | 26 +++++++++++++++++++++-----
> >  virt/kvm/arm/vgic/vgic-mmio-v3.c | 31 ++++++++++++++++++++++++-------
> >  virt/kvm/arm/vgic/vgic-mmio.c    | 18 +++++++++++++-----
> >  virt/kvm/arm/vgic/vgic-mmio.h    | 19 +++++++++++--------
> >  4 files changed, 69 insertions(+), 25 deletions(-)
> >
> > diff --git a/virt/kvm/arm/vgic/vgic-mmio-v2.c b/virt/kvm/arm/vgic/vgic-mmio-v2.c
> > index 34e36fc..b79de42 100644
> > --- a/virt/kvm/arm/vgic/vgic-mmio-v2.c
> > +++ b/virt/kvm/arm/vgic/vgic-mmio-v2.c
> > @@ -77,11 +77,26 @@ static void vgic_mmio_write_v2_misc(struct kvm_vcpu *vcpu,
> >     }
> >  }
> >
> > -static void vgic_mmio_uaccess_write_v2_group(struct kvm_vcpu *vcpu,
> > -                                        gpa_t addr, unsigned int len,
> > -                                        unsigned long val)
> > +static int vgic_mmio_uaccess_write_v2_misc(struct kvm_vcpu *vcpu,
> > +                                      gpa_t addr, unsigned int len,
> > +                                      unsigned long val)
> > +{
> > +   switch (addr & 0x0c) {
> > +   case GIC_DIST_IIDR:
> > +           if (val != vgic_mmio_read_v2_misc(vcpu, addr, len))
> > +                   return -EINVAL;
> > +   }
> > +
> > +   vgic_mmio_write_v2_misc(vcpu, addr, len, val);
> > +   return 0;
> > +}
> > +
> > +static int vgic_mmio_uaccess_write_v2_group(struct kvm_vcpu *vcpu,
> > +                                       gpa_t addr, unsigned int len,
> > +                                       unsigned long val)
> >  {
> >     /* Ignore writes from userspace */
> > +   return 0;
> >  }
>
> I think it'd make more sense if this patch came before the previous one
> (change the prototype of the userspace accessor, and only then add a new
> accessor that can return an error). It would avoid churn in the above
> function, and you could move vgic_mmio_uaccess_write_v2_misc into patch 6.

The intention was that the first six patches dealt with the basic
interrupt grouping support, pretending that we don't have this
save/restore issue with GICv2, and then let the following patches deal
with the GICv2 problem.

>
> Not a big deal though, as the code is otherwise perfectly fine.
>

That said, I'm happy to try to rework this if you think the git history
will be easier to carry as a result.

Thanks,
-Christoffer
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