Re: [PATCH 1/1] direct mmio for passthrough - kernel part

Anthony Liguori
2008-04-01T15:20:49Z

Anthony Liguori wrote:
> Avi Kivity wrote:
>> benami@il.ibm.com wrote:
>>
>>> From: Ben-Ami Yassour <benami@il.ibm.com>
>>>
>>> Enable a guest to access a device's memory mapped I/O regions directly.
>>> Userspace sends the mmio regions that the guest can access. On the
>>> first
>>> page fault for an access to an mmio address the host translates the
>>> gva to hpa,
>>> and updates the sptes.
>>>
>>>
>>
>> Can you explain why you're not using the regular memory slot
>> mechanism? i.e. have userspace mmap(/dev/mem) and create a memslot
>> containing that at the appropriate guest physical address?
>>
>
> /dev/mem is often restricted in what memory can be mapped. However,
> we can't add something like this to KVM that allows arbitrary HPA's to
> be mapped into a guest from userspace. This is just as bad as
> /dev/mem and is going to upset a lot of people.
>
> Regardless of whether we can use /dev/mem, I think we should introduce
> a new char device anyway. We only need to mmap() MMIO regions which
> are mapped by the PCI bus, presumably, the kernel should know about
> these mappings. The driver should only allow mappings that are valid
> for a particular PCI device such that it cannot be abused to map
> arbitrary regions of memory into a guest. Bonus points if it can
> validate that there isn't a valid Linux driver loaded for the given
> PCI device.

Which is apparently entirely unnecessary as we already have
/sys/bus/pci/.../region. It's just a matter of checking if a vma is
VM_IO and then dealing with the subsequent reference counting issues as
Avi points out.

Regards,

Anthony Liguori

> Regards,
>
> Anthony Liguori
>
>> There are some issues with refcounting, but Andrea has some tricks to
>> deal with that.
>>
>>