linux-kernel - Re: [PATCH 5/5] KVM: Allow host IRQ sharing for passed-through PCI 2.3 devices

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Message-ID: <4CFD108F.5000200@siemens.com>
Date:	Mon, 06 Dec 2010 17:34:23 +0100
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Jan Kiszka <jan.kiszka@....de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, Tom Lyon <pugs@...co.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH 5/5] KVM: Allow host IRQ sharing for passed-through PCI
 2.3 devices

Am 06.12.2010 17:21, Avi Kivity wrote:
> On 12/04/2010 01:37 AM, Jan Kiszka wrote:
>> From: Jan Kiszka<jan.kiszka@...mens.com>
>>
>> PCI 2.3 allows to generically disable IRQ sources at device level. This
>> enables us to share IRQs of such devices on the host side when passing
>> them to a guest.
>>
>> However, IRQ disabling via the PCI config space is more costly than
>> masking the line via disable_irq. Therefore we register an IRQ sharing
>> notifier and switch between line and device level disabling on demand.
>>
>> This feature is optional, user space has to request it explicitly as it
>> also has to inform us about its view of PCI_COMMAND_INTX_DISABLE. That
>> way, we can avoid unmasking the interrupt and signaling it if the guest
>> masked it via the PCI config space.
>>
>>
>> diff --git a/Documentation/kvm/api.txt b/Documentation/kvm/api.txt
>> index e1a9297..5e164db 100644
>> --- a/Documentation/kvm/api.txt
>> +++ b/Documentation/kvm/api.txt
>> @@ -1112,6 +1112,14 @@ following flags are specified:
>>
>>   /* Depends on KVM_CAP_IOMMU */
>>   #define KVM_DEV_ASSIGN_ENABLE_IOMMU	(1<<  0)
>> +/* The following two depend on KVM_CAP_PCI_2_3 */
>> +#define KVM_DEV_ASSIGN_PCI_2_3		(1<<  1)
>> +#define KVM_DEV_ASSIGN_MASK_INTX	(1<<  2)
>> +
>> +If KVM_DEV_ASSIGN_PCI_2_3 is set, the kernel will manage legacy INTx interrupts
>> +via the PCI-2.3-compliant device-level mask, but only if IRQ sharing with other
>> +assigned or host devices requires it. KVM_DEV_ASSIGN_MASK_INTX specifies the
>> +guest's view on the INTx mask, see KVM_ASSIGN_SET_INTX_MASK for details.
>>
>>   4.48 KVM_DEASSIGN_PCI_DEVICE
>>
>> @@ -1263,6 +1271,23 @@ struct kvm_assigned_msix_entry {
>>   	__u16 padding[3];
>>   };
>>
>> +4.54 KVM_ASSIGN_SET_INTX_MASK
>> +
>> +Capability: KVM_CAP_PCI_2_3
>> +Architectures: x86
>> +Type: vm ioctl
>> +Parameters: struct kvm_assigned_pci_dev (in)
>> +Returns: 0 on success, -1 on error
>> +
>> +Informs the kernel about the guest's view on the INTx mask. As long as the
>> +guest masks the legacy INTx, the kernel will refrain from unmasking it at
>> +hardware level and will not assert the guest's IRQ line. User space is still
>> +responsible for applying this state to the assigned device's real config space.
> 
> What's the protocol for doing this?  I suppose userspace has to disable 
> interrupts, ioctl(SET_INTX_MASK, masked), ..., ioctl(SET_INTX_MASK, 
> unmasked), enable interrupts?

Userspace just has to synchronize against itself - what it already does:
qemu_mutex, and masking/unmasking is synchronous /wrt the the executing
VCPU. Otherwise, masking/unmasking is naturally racy, also in Real Life.
The guest resolves the remaining races.

> 
> Isn't there a race window between the two operations?
> 
> Maybe we should give the kernel full ownership of that bit.

I think this is what VFIO does and is surely cleaner than this approach.
But it's not possible with the existing interface (sysfs + KVM ioctls) -
or can you restrict the sysfs access to the config space in such details?

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux
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