Re: [RFC PATCH 1/4] ARM: KVM: on unhandled IO mem abort, route the call to the KVM MMIO bus

From: Eric Auger
Date: Thu Nov 13 2014 - 11:08:15 EST

On 11/13/2014 04:31 PM, Andre Przywara wrote:
>
>
> On 13/11/14 15:02, Nikolay Nikolaev wrote:
>> On Thu, Nov 13, 2014 at 4:23 PM, Eric Auger <eric.auger@xxxxxxxxxx> wrote:
>>> On 11/13/2014 03:16 PM, Eric Auger wrote:
>>>> On 11/13/2014 11:45 AM, Nikolay Nikolaev wrote:
>>>>> On Mon, Nov 10, 2014 at 6:27 PM, Christoffer Dall
>>>>> <christoffer.dall@xxxxxxxxxx> wrote:
>>>>>> On Mon, Nov 10, 2014 at 05:09:07PM +0200, Nikolay Nikolaev wrote:
>>>>>>> Hello,
>>>>>>>
>>>>>>> On Fri, Mar 28, 2014 at 9:09 PM, Christoffer Dall
>>>>>>> <christoffer.dall@xxxxxxxxxx> wrote:
>>>>>>>>
>>>>>>>> On Thu, Mar 13, 2014 at 04:57:26PM +0100, Antonios Motakis wrote:
>>>>>>>>> On an unhandled IO memory abort, use the kvm_io_bus_* API in order to
>>>>>>>>> handle the MMIO access through any registered read/write callbacks. This
>>>>>>>>> is a dependency for eventfd support (ioeventfd and irqfd).
>>>>>>>>>
>>>>>>>>> However, accesses to the VGIC are still left implemented independently,
>>>>>>>>> since the kvm_io_bus_* API doesn't pass the VCPU pointer doing the access.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Antonios Motakis <a.motakis@xxxxxxxxxxxxxxxxxxxxxx>
>>>>>>>>> Signed-off-by: Nikolay Nikolaev <n.nikolaev@xxxxxxxxxxxxxxxxxxxxxx>
>>>>>>>>> ---
>>>>>>>>> arch/arm/kvm/mmio.c | 32 ++++++++++++++++++++++++++++++++
>>>>>>>>> virt/kvm/arm/vgic.c | 5 ++++-
>>>>>>>>> 2 files changed, 36 insertions(+), 1 deletion(-)
>>>>>>>>>
>>>>>>>>> diff --git a/arch/arm/kvm/mmio.c b/arch/arm/kvm/mmio.c
>>>>>>>>> index 4cb5a93..1d17831 100644
>>>>>>>>> --- a/arch/arm/kvm/mmio.c
>>>>>>>>> +++ b/arch/arm/kvm/mmio.c
>>>>>>>>> @@ -162,6 +162,35 @@ static int decode_hsr(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>>>>>>>> return 0;
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>> +/**
>>>>>>>>> + * kvm_handle_mmio - handle an in-kernel MMIO access
>>>>>>>>> + * @vcpu: pointer to the vcpu performing the access
>>>>>>>>> + * @run: pointer to the kvm_run structure
>>>>>>>>> + * @mmio: pointer to the data describing the access
>>>>>>>>> + *
>>>>>>>>> + * returns true if the MMIO access has been performed in kernel space,
>>>>>>>>> + * and false if it needs to be emulated in user space.
>>>>>>>>> + */
>>>>>>>>> +static bool handle_kernel_mmio(struct kvm_vcpu *vcpu, struct kvm_run *run,
>>>>>>>>> + struct kvm_exit_mmio *mmio)
>>>>>>>>> +{
>>>>>>>>> + int ret;
>>>>>>>>> + if (mmio->is_write) {
>>>>>>>>> + ret = kvm_io_bus_write(vcpu->kvm, KVM_MMIO_BUS, mmio->phys_addr,
>>>>>>>>> + mmio->len, &mmio->data);
>>>>>>>>> +
>>>>>>>>> + } else {
>>>>>>>>> + ret = kvm_io_bus_read(vcpu->kvm, KVM_MMIO_BUS, mmio->phys_addr,
>>>>>>>>> + mmio->len, &mmio->data);
>>>>>>>>> + }
>>>>>>>>> + if (!ret) {
>>>>>>>>> + kvm_prepare_mmio(run, mmio);
>>>>>>>>> + kvm_handle_mmio_return(vcpu, run);
>>>>>>>>> + }
>>>>>>>>> +
>>>>>>>>> + return !ret;
>>>>>>>>> +}
>>>>>>>>> +
>>>>>>>>> int io_mem_abort(struct kvm_vcpu *vcpu, struct kvm_run *run,
>>>>>>>>> phys_addr_t fault_ipa)
>>>>>>>>> {
>>>>>>>>> @@ -200,6 +229,9 @@ int io_mem_abort(struct kvm_vcpu *vcpu, struct kvm_run *run,
>>>>>>>>> if (vgic_handle_mmio(vcpu, run, &mmio))
>>>>>>>>> return 1;
>>>>>>>>>
>>>>>>>>> + if (handle_kernel_mmio(vcpu, run, &mmio))
>>>>>>>>> + return 1;
>>>>>>>>> +
>>>>>>>
>>>>>>>
>>>>>>> We're reconsidering ioeventfds patchseries and we tried to evaluate
>>>>>>> what you suggested here.
>>>>>>>
>>>>>>>>
>>>>>>>> this special-casing of the vgic is now really terrible. Is there
>>>>>>>> anything holding you back from doing the necessary restructure of the
>>>>>>>> kvm_bus_io_*() API instead?
>>>>>>>
>>>>>>> Restructuring the kvm_io_bus_ API is not a big thing (we actually did
>>>>>>> it), but is not directly related to the these patches.
>>>>>>> Of course it can be justified if we do it in the context of removing
>>>>>>> vgic_handle_mmio and leaving only handle_kernel_mmio.
>>>>>>>
>>>>>>>>
>>>>>>>> That would allow us to get rid of the ugly
>>>>>>>> Fix it! in the vgic driver as well.
>>>>>>>
>>>>>>> Going through the vgic_handle_mmio we see that it will require large
>>>>>>> refactoring:
>>>>>>> - there are 15 MMIO ranges for the vgic now - each should be
>>>>>>> registered as a separate device
>>> Re-correcting Andre's address, sorry:
>>> Hi Nikolay, Andre,
>>>
>>> what does mandate to register 15 devices? Isn't possible to register a
>>> single kvm_io_device covering the whole distributor range [base, base +
>>> KVM_VGIC_V2_DIST_SIZE] (current code) and in associated
>>> kvm_io_device_ops read/write locate the addressed range and do the same
>>> as what is done in current vgic_handle_mmio? Isn't it done that way for
>>
>> Well, then we'll actually get slower mmio processing. Instead of calling
>> vgic_handle_mmio in io_mem_abort, we'll be calling kvm_io_bus_write.
>> This just adds another level of translation (i.e. find the kvm_io_ device)
>> and the underlying vgic code will remain almost the same.
>
> Agreed. That was one possibility I came around also, but I think it
> defeats the purpose of the rework, which is mostly to get rid of the
> GIC's private MMIO dispatching code, right?
Hi Andre, Nikolay,

Well my feeling is one of your primary goal was to split in a clean way
v2 and v3 MMIO regions. This may be achieved by creating either
io_device. Also in the future don't we have ITS & redistributors
showing up that could match io_devices too - but all that stuff still
are very fuzzy for me ;-) - ?

with respect to vcpu, we could add an opaque void * in kvm_io_device and
pass the vcpu when calling kvm_iodevice_init().

I also think we must resume integration of ioeventfd, whatever the
chosen solution all the more so there seems to be some momentum on this now.

Best Regards

Eric
>
> But honestly I would happily sacrifice "performance" for easier VGIC
> code - especially if one thinks about security for instance. Though I
> think that another memory reference doesn't really matter in this
> context ;-)
>
>>> the ioapic? what do I miss?
>> I looked quickly in the ioapic code, and if I get it right there are no "ranges'
>> like what we have with the GIC. They have this regselect/regwindow concept
>> and they seem to have much less "registers" to handle. GIC seems a lot more
>> complex in terms of MMIO interface.
>
> Right, that was my impression, too. IOAPIC isn't really comparable to
> the GIC in this respect. That's why I was going away from this rework,
> since I thought that the kvm_io_bus API wasn't really meant for such
> beasts as the GIC.
>
> Cheers,
> Andre.
>
>>
>> regards,
>> Nikolay Nikolaev
>>
>>>
>>> Thanks
>>>
>>> Best Regards
>>>
>>> Eric
>>>>>>> - the handler of each range should be split into read and write
>>>>>>> - all handlers take 'struct kvm_exit_mmio', and pass it to
>>>>>>> 'vgic_reg_access', 'mmio_data_read' and 'mmio_data_read'
>>>>>>>
>>>>>>> To sum up - if we do this refactoring of vgic's MMIO handling +
>>>>>>> kvm_io_bus_ API getting 'vcpu" argument we'll get a 'much' cleaner
>>>>>>> vgic code and as a bonus we'll get 'ioeventfd' capabilities.
>>>>>>>
>>>>>>> We have 3 questions:
>>>>>>> - is the kvm_io_bus_ getting 'vcpu' argument acceptable for the other
>>>>>>> architectures too?
>>>>>>> - is this huge vgic MMIO handling redesign acceptable/desired (it
>>>>>>> touches a lot of code)?
>>>>>>> - is there a way that ioeventfd is accepted leaving vgic.c in it's
>>>>>>> current state?
>>>>>>>
>>>>>> Not sure how the latter question is relevant to this, but check with
>>>>>> Andre who recently looked at this as well and decided that for GICv3 the
>>>>>> only sane thing was to remove that comment for the gic.
>>>>> @Andre - what's your experience with the GICv3 and MMIO handling,
>>>>> anything specific?
>>>>>>
>>>>>> I don't recall the details of what you were trying to accomplish here
>>>>>> (it's been 8 months or so) but the surely the vgic handling code should
>>>>>> *somehow* be integrated into the handle_kernel_mmio (like Paolo
>>>>>> suggested), unless you come back and tell me that that would involve a
>>>>>> complete rewrite of the vgic code.
>>>>> I'm experimenting now - it's not exactly rewrite of whole vgic code,
>>>>> but it will touch a lot of it - all MMIO access handlers and the
>>>>> supporting functions.
>>>>> We're ready to spend the effort. My question is - is this acceptable?
>>>>>
>>>>> regards,
>>>>> Nikolay Nikolaev
>>>>> Virtual Open Systems
>>>>>>
>>>>>> -Christoffer
>>>>> _______________________________________________
>>>>> kvmarm mailing list
>>>>> kvmarm@xxxxxxxxxxxxxxxxxxxxx
>>>>> https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
>>>>>
>>>>
>>>
>>

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