Re: [PATCH 08/12] KVM: arm/arm64: implement kvm_io_bus MMIO handling for the VGIC

Christoffer Dall
2015-03-20T14:25:41Z

On Fri, Mar 20, 2015 at 12:40:02PM +0000, Andre Przywara wrote:
> On 03/19/2015 03:44 PM, Andre Przywara wrote:
> > Hej Christoffer,
> >
>
> [ ... ]
>
> >>> +static int vgic_handle_mmio_access(struct kvm_vcpu *vcpu,
> >>> + struct kvm_io_device *this, gpa_t addr,
> >>> + int len, void *val, bool is_write)
> >>> +{
> >>> + struct vgic_dist *dist = &vcpu->kvm->arch.vgic;
> >>> + struct vgic_io_device *iodev = container_of(this,
> >>> + struct vgic_io_device, dev);
> >>> + struct kvm_run *run = vcpu->run;
> >>> + const struct vgic_io_range *range;
> >>> + struct kvm_exit_mmio mmio;
> >>> + bool updated_state;
> >>> + gpa_t offset;
> >>> +
> >>> + offset = addr - iodev->addr;
> >>> + range = vgic_find_range(iodev->reg_ranges, len, offset);
> >>> + if (unlikely(!range || !range->handle_mmio)) {
> >>> + pr_warn("Unhandled access %d %08llx %d\n", is_write, addr, len);
> >>> + return -ENXIO;
> >>> + }
> >>> +
> >>> + mmio.phys_addr = addr;
> >>> + mmio.len = len;
> >>> + mmio.is_write = is_write;
> >>> + if (is_write)
> >>> + memcpy(mmio.data, val, len);
> >>> + mmio.private = iodev->redist_vcpu;
> >>> +
> >>> + spin_lock(&dist->lock);
> >>> + offset -= range->base;
> >>> + if (vgic_validate_access(dist, range, offset)) {
> >>> + updated_state = call_range_handler(vcpu, &mmio, offset, range);
> >>> + if (!is_write)
> >>> + memcpy(val, mmio.data, len);
> >>> + } else {
> >>> + if (!is_write)
> >>> + memset(val, 0, len);
> >>> + updated_state = false;
> >>> + }
> >>> + spin_unlock(&dist->lock);
> >>> + kvm_prepare_mmio(run, &mmio);
> >>
> >> we're not the only user of kvm_exit_mmio I believe, so we could rename
> >
> > (assuming you mean we _are_ the only user here, which I can acknowledge)
> >
> >> this to vgic_io as well and you could change the mmio.data array to be a
> >> void *val pointer, which just gets set to the pointer passed into this
> >> function (which I think points to the kvm_run structs data array) and
> >> you can avoid all these memcopies, right?
> >
> > That sounds indeed tempting, but the comment on the struct kvm_exit_mmio
> > declaration reads:
> > /*
> > * The in-kernel MMIO emulation code wants to use a copy of run->mmio,
> > * which is an anonymous type. Use our own type instead.
> > */
> > How I understand this the structure was introduced to _not_ use the same
> > memory, but use a copy instead. Do you remember any reason for this? And
> > in how far is this type anonymous? It's even in an uapi header.
> >
> > Briefly looking at the code we do quite some memcpy on the way.
> > I am about to go all the way down into that ARM MMIO handling cave now
> > to check this (Marc, if I am not showing up again after some hours,
> > please come and rescue me ;-)
>
> So, I feel that there is quite some unneeded copying and masking on the
> way, but a real fix would be quite invasive and needs quite some testing
> and review. I don't feel like rushing this into a v2 of this series.
> I quickly did what you proposed (replacing memcpy by pointer
> assignment), and that seems to work, but I don't have many chances of
> testing this this weekend, since I am on the road. Also I have to dig
> out my cross-endian test scripts first. So not sure if you want to take
> the risk with this series.
> I changed the other minor points you mentioned in the review though, so
> do you want to have a "v1.5" or how do we proceed from here?
>
Why is it so invasive? See my previous mail. If you can convince me
that we're touching something truly nasty (code snippet?) then ok, we
can take a version without the cleanup and cleanup later.

If you're out of time, send me whatever you have, and we'll see if I fix
it up further or just leave it for the future.

Thanks,
-Christoffer