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

From: [email-masked] (Eric Auger)
Date: Thu, 13 Nov 2014 15:16:16 +0100

On 11/13/2014 11:45 AM, Nikolay Nikolaev wrote:
> On Mon, Nov 10, 2014 at 6:27 PM, Christoffer Dall
> <[email-masked]> 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
>>> <[email-masked]> 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 <[email-masked]>
>>>>> Signed-off-by: Nikolay Nikolaev <[email-masked]>
>>>>> ---
>>>>>  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
Hi Nikolaev, 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
the ioapic? what do I miss?

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
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