Re: [RFC PATCH 12/45] KVM: arm/arm64: vgic-new: Add MMIO handling framework

Marc Zyngier
2016-04-12T15:56:08Z

On 12/04/16 13:50, Christoffer Dall wrote:
> On Mon, Apr 11, 2016 at 11:53:21AM +0100, Andre Przywara wrote:
>> Hi Christoffer,
>>
>> On 31/03/16 10:08, Christoffer Dall wrote:
>>> Hi Andre,
>>>
>>> [cc'ing Paolo here for his thoughts on the KVM IO bus framework]
>>>
>>> On Fri, Mar 25, 2016 at 02:04:35AM +0000, Andre Przywara wrote:
>>>> We register each register group of the distributor and redistributors
>>>> as separate regions of the kvm-io-bus framework. This way calls get
>>>> directly handed over to the actual handler.
>>>> This puts a lot more regions into kvm-io-bus than what we use at the
>>>> moment on other architectures, so we will probably need to revisit the
>>>> implementation of the framework later to be more efficient.
>>>
>>> Looking more carefully at the KVM IO bus stuff, it looks like it is
>>> indeed designed to be a *per device* thing you register, not a *per
>>> register* thing.
>>>
>>> My comments to Vladimir's bug report notwithstanding, there's still a
>>> choice here to:
>>>
>>> 1) Expand/modify the KVM IO bus framework to take an arbitrary number of devices
>>>
>>> 2) Build a KVM architectureal generic framework on top of the IO bus
>>> framework to handle individual register regions.
>>>
>>> 3) Stick with what we had before, do not modify the KVM IO bus stuff,
>>> and handle the individual register region business locally within the
>>> arm/vgic code.
>>>
>>>
>>>>
>>>> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
>>>> Signed-off-by: Eric Auger <eric.auger@linaro.org>
>>>> ---
>>>> include/kvm/vgic/vgic.h | 9 ++
>>>> virt/kvm/arm/vgic/vgic_mmio.c | 194 ++++++++++++++++++++++++++++++++++++++++++
>>>> virt/kvm/arm/vgic/vgic_mmio.h | 47 ++++++++++
>>>> 3 files changed, 250 insertions(+)
>>>> create mode 100644 virt/kvm/arm/vgic/vgic_mmio.c
>>>> create mode 100644 virt/kvm/arm/vgic/vgic_mmio.h
>>>>
>>>> diff --git a/include/kvm/vgic/vgic.h b/include/kvm/vgic/vgic.h
>>>> index 2ce9b4a..a8262c7 100644
>>>> --- a/include/kvm/vgic/vgic.h
>>>> +++ b/include/kvm/vgic/vgic.h
>>>> @@ -106,6 +106,12 @@ struct vgic_irq {
>>>> enum vgic_irq_config config; /* Level or edge */
>>>> };
>>>>
>>>> +struct vgic_io_device {
>>>> + gpa_t base_addr;
>>>> + struct kvm_vcpu *redist_vcpu;
>>>> + struct kvm_io_device dev;
>>>> +};
>>>> +
>>>> struct vgic_dist {
>>>> bool in_kernel;
>>>> bool ready;
>>>> @@ -132,6 +138,9 @@ struct vgic_dist {
>>>> u32 enabled;
>>>>
>>>> struct vgic_irq *spis;
>>>> +
>>>> + struct vgic_io_device *dist_iodevs;
>>>> + struct vgic_io_device *redist_iodevs;
>>>> };
>>>>
>>>> struct vgic_v2_cpu_if {
>>>> diff --git a/virt/kvm/arm/vgic/vgic_mmio.c b/virt/kvm/arm/vgic/vgic_mmio.c
>>>> new file mode 100644
>>>> index 0000000..26c46e7
>>>> --- /dev/null
>>>> +++ b/virt/kvm/arm/vgic/vgic_mmio.c
>>>> @@ -0,0 +1,194 @@
>>>> +/*
>>>> + * VGIC MMIO handling functions
>>>> + *
>>>> + * This program is free software; you can redistribute it and/or modify
>>>> + * it under the terms of the GNU General Public License version 2 as
>>>> + * published by the Free Software Foundation.
>>>> + *
>>>> + * This program is distributed in the hope that it will be useful,
>>>> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
>>>> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
>>>> + * GNU General Public License for more details.
>>>> + */
>>>> +
>>>> +#include <linux/kvm.h>
>>>> +#include <linux/kvm_host.h>
>>>> +#include <kvm/iodev.h>
>>>> +#include <kvm/vgic/vgic.h>
>>>> +#include <linux/bitops.h>
>>>> +#include <linux/irqchip/arm-gic.h>
>>>> +
>>>> +#include "vgic.h"
>>>> +#include "vgic_mmio.h"
>>>> +
>>>> +void write_mask32(u32 value, int offset, int len, void *val)
>>>> +{
>>>> + value = cpu_to_le32(value) >> (offset * 8);
>>>> + memcpy(val, &value, len);
>>>> +}
>>>> +
>>>> +u32 mask32(u32 origvalue, int offset, int len, const void *val)
>>>> +{
>>>> + origvalue &= ~((BIT_ULL(len) - 1) << (offset * 8));
>>>> + memcpy((char *)&origvalue + (offset * 8), val, len);
>>>> + return origvalue;
>>>> +}
>>>> +
>>>> +#ifdef CONFIG_KVM_ARM_VGIC_V3
>>>> +void write_mask64(u64 value, int offset, int len, void *val)
>>>> +{
>>>> + value = cpu_to_le64(value) >> (offset * 8);
>>>> + memcpy(val, &value, len);
>>>> +}
>>>> +
>>>> +/* FIXME: I am clearly misguided here, there must be some saner way ... */
>>>
>>> I'm confuses in general. Can you explain what these mask functions do
>>> overall at some higher level?
>>
>> They do what you already guessed: take care about masking and endianness.
>>
>>> I also keep having a feeling that mixing endianness stuff into the
>>> emulation code itself is the wrong way to go about it.
>>
>> I agree.
>>
>>> The emulation
>>> code should just deal with register values of varying length and the
>>> interface to the VGIC should abstract all endianness nonsense for us,
>>> but I also think I've lost this argument some time in the past. Sigh.
>
> I tend to agree with this. Who did you loose this argument against and
> do you have a pointer?
>
>>>
>>> But, is the maximum read/write unit for any MMIO access not a 64-bit
>>> value? So why can't we let the VGIC emulation code simply take/return a
>>> u64 which is then masked off/morphed into the right endianness outside
>>> the VGIC code?
>>
>> The main problem is that the interface for kvm_io_bus is (int len, void
>> *val).
>
> That could be solved by always just doing a (u64) conversion on the
> caller/receiver side. E.g.
>
> int vgic_handle_mmio_access(..., int len, void *val, bool is_write)
> {
> u64 data;
>
> if (unsupported_vgic_len(len))
> return -ENXIO;
>
> if (is_write)
> data = mmio_read_buf(val, len);
>
> // data is now just some data of some lenght in a typed register
>
> call_range_handler(vcpu, &data, len, ...);
>
> if (!is_write)
> mmio_write_buf(val, len, data);
> }
>
> (and share the mmio_ functions in a header file between mmio.c and
> consumers of the packed data).
>
> The idea would be that the gic is just emulation code in C, which can be
> compiled to some endianness, and we really don't care about how it's
> physically stored in memory.
>
>
>
>> And since the guest's MMIO access can be of different length, we need to
>> take care of this in any case (since we need to know how many IRQs we
>> need to update). So we cannot get rid of the length parameter.
>
> no we cannot, but we can use a typed pointer instead of a void pointer
> everywhere in the vgic code, since the max access we're going to support
> is 64 bits.
>
>> I guess what we could do is to either:
>> a) Declare the VGIC as purely little endian (which it really is, AFAIK).
>> We care about the necessary endianness conversion in mmio.c before
>> invoking the kvm_io_bus framework. But that means that we need to deal
>> with the _host's_ endianness in the VGIC emulation.
>> I think this is very close to what we currently do. OR
>> b) Declare our VGIC emulation as always using the host's endianess. We
>> wouldn't need to care about endianness in the VGIC code anymore (is that
>> actually true?) We convert the MMIO traps (which are little endian
>> always) into the host's endianness before passing it on to kvm-io-bus.
>>
>> I just see that this probably adds more to the confusion, oh well...
>
> No, it doesn't add anymore confusion. I think option (b) is what we
> should do, unless it's not possible for some reason that I fail to
> realize at this very moment.
>
> If all your pointers in the vgic emulation code are always typed, then I
> don't see why you'd have to worry about endianness anywhere?
>
> The only place we should worry about endianness is the
> vcpu_data_guest_to_host() and vcpu_data_host_to_guest() functions.
> Doing it anywhere else as well is an indication that we're doing
> something wrong.

That seems sensible to me. This should define a frontier where the
access to the GIC is always expected in LE mode, but we implement the
GIC using the host endianness. We have to make sure that no data
structure gets passed outside of this interface (and that includes
userspace access).

This works fine for MMIO, but I'm more worried of memory-based
interfaces that we get with GICv3 and the ITS, specially with migration.
Property table is fine (byte access), but pending table can be slightly
more tricky (bit position in words). And then we get to the ITS tables,
which need a standard representation. Maybe it is too early for that,
but it is worth keeping it in mind.

Thanks,

M.
--
Jazz is not dead. It just smells funny...