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

[Anthony Liguori <anthony@codemonkey.ws>]

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.
>>
>>   
>


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