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

Andre Przywara
2015-03-20T12:33:47Z

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?

Cheers,
Andre.