Subject: [PATCH] kvmtool: don't use PCI config space IRQ line field

From: [email-masked] (Andre Przywara)
Date: Mon, 15 Jun 2015 11:45:38 +0100

On 06/05/2015 05:41 PM, Will Deacon wrote:
> On Thu, Jun 04, 2015 at 04:20:45PM +0100, Andre Przywara wrote:

Hi Will,

sorry, almost forgot about this email...

>> In PCI config space there is an interrupt line field (offset 0x3f),
>> which is used to initially communicate the IRQ line number from
>> firmware to the OS. _Hardware_ should never use this information,
>> as the OS is free to write any information in there.
>> But kvmtool uses this number when it triggers IRQs in the guest,
>> which fails starting with Linux 3.19-rc1, where the PCI layer starts
>> writing the virtual IRQ number in there.
>>
>> Fix that by storing the IRQ number in a separate field in
>> struct virtio_pci, which is independent from the PCI config space
>> and cannot be influenced by the guest.
>> This fixes ARM/ARM64 guests using PCI with newer kernels.
>>
>> Signed-off-by: Andre Przywara <[email-masked]>
>> ---
>>  include/kvm/virtio-pci.h | 8 ++++++++
>>  virtio/pci.c             | 9 ++++++---
>>  2 files changed, 14 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/kvm/virtio-pci.h b/include/kvm/virtio-pci.h
>> index c795ce7..b70cadd 100644
>> --- a/include/kvm/virtio-pci.h
>> +++ b/include/kvm/virtio-pci.h
>> @@ -30,6 +30,14 @@ struct virtio_pci {
>>  	u8			isr;
>>  	u32			features;
>>  
>> +	/*
>> +	 * We cannot rely on the INTERRUPT_LINE byte in the config space once
>> +	 * we have run guest code, as the OS is allowed to use that field
>> +	 * as a scratch pad to communicate between driver and PCI layer.
>> +	 * So store our legacy interrupt line number in here for internal use.
>> +	 */
>> +	u8			legacy_irq_line;
>> +
>>  	/* MSI-X */
>>  	u16			config_vector;
>>  	u32			config_gsi;
>> diff --git a/virtio/pci.c b/virtio/pci.c
>> index 7556239..e17e5a9 100644
>> --- a/virtio/pci.c
>> +++ b/virtio/pci.c
>> @@ -141,7 +141,7 @@ static bool virtio_pci__io_in(struct ioport *ioport, struct kvm_cpu *vcpu, u16 p
>>  		break;
>>  	case VIRTIO_PCI_ISR:
>>  		ioport__write8(data, vpci->isr);
>> -		kvm__irq_line(kvm, vpci->pci_hdr.irq_line, VIRTIO_IRQ_LOW);
>> +		kvm__irq_line(kvm, vpci->legacy_irq_line, VIRTIO_IRQ_LOW);
>>  		vpci->isr = VIRTIO_IRQ_LOW;
>>  		break;
>>  	default:
>> @@ -299,7 +299,7 @@ int virtio_pci__signal_vq(struct kvm *kvm, struct virtio_device *vdev, u32 vq)
>>  			kvm__irq_trigger(kvm, vpci->gsis[vq]);
>>  	} else {
>>  		vpci->isr = VIRTIO_IRQ_HIGH;
>> -		kvm__irq_trigger(kvm, vpci->pci_hdr.irq_line);
>> +		kvm__irq_trigger(kvm, vpci->legacy_irq_line);
>>  	}
>>  	return 0;
>>  }
>> @@ -323,7 +323,7 @@ int virtio_pci__signal_config(struct kvm *kvm, struct virtio_device *vdev)
>>  			kvm__irq_trigger(kvm, vpci->config_gsi);
>>  	} else {
>>  		vpci->isr = VIRTIO_PCI_ISR_CONFIG;
>> -		kvm__irq_trigger(kvm, vpci->pci_hdr.irq_line);
>> +		kvm__irq_trigger(kvm, vpci->legacy_irq_line);
>>  	}
>>  
>>  	return 0;
>> @@ -422,6 +422,9 @@ int virtio_pci__init(struct kvm *kvm, void *dev, struct virtio_device *vdev,
>>  	if (r < 0)
>>  		goto free_msix_mmio;
>>  
>> +	/* save the IRQ that device__register() has allocated */
>> +	vpci->legacy_irq_line = vpci->pci_hdr.irq_line;
> 
> I'd rather we used the container_of trick that we do for virtio-mmio
> devices when assigning the irq in device__register. Then we can avoid
> this line completely.

Not completely sure I get what you mean, I take it you want to assign
legacy_irq_line in pci__assign_irq() directly (where the IRQ number is
allocated).
But this function is PCI generic code and is used by the VESA
framebuffer and the shmem device on x86 as well. For those devices
dev_hdr is not part of a struct virtio_pci, so we can't do container_of
to assign the legacy_irq_line here directly.
Admittedly this fix should apply to the other two users as well, but
VESA does not use interrupts and pci-shmem is completely broken anyway,
so I didn't bother to fix it in this regard.
Would it be justified to provide an IRQ number field in struct
device_header to address all users?

Or what am I missing here?

Cheers,
Andre