Re: [PATCH v2 11/16] KVM: introduce a 'mmap' method for KVM devices

[Paolo Bonzini <pbonzini@redhat.com>]

On 25/02/19 11:57, Cédric Le Goater wrote:
> Hello Paolo,
> 
> On 2/25/19 4:33 AM, David Gibson wrote:
>> On Fri, Feb 22, 2019 at 12:28:35PM +0100, Cédric Le Goater wrote:
>>> Some KVM devices will want to handle special mappings related to the
>>> underlying HW. For instance, the XIVE interrupt controller of the
>>> POWER9 processor has MMIO pages for thread interrupt management and
>>> for interrupt source control that need to be exposed to the guest when
>>> the OS has the required support.
>>>
>>> Signed-off-by: Cédric Le Goater <clg@kaod.org>
>>
>> Ah, when I suggested mmap() on the base device fd, I hadn't realized
>> there wasn't a facility for that yet.
>>
>> Have you discussed this with Paolo?  
> 
> Not yet.
> 
>> We'll need some core KVM buy in to merge this.
> 
> Here is an extension of the KVM device to allow special mappings.
> Something we would need for the support of the POWER9 XIVE interrupt 
> controller.
> 
> There are two MMIOs we need to expose to the guest : 
> 
>  1. HW MMIO controlling of the interrupt presenter registers (TIMA)
>  2. HW MMIO of the interrupt sources for interrupt management (ESB)
> 
> The TIMA could have been exposed with a page offset in the vCPU mapping
> but as it only makes sense when the XIVE interrupt mode is active, we 
> chose to use directly the KVM device fd for that. Is that ok ? 
> 
> An alternate solution is to use a device ioctl to allocate an anon fd
> and do the mapping, but that seems like extra fuss for the same result. 

It's okay, it's a natural extension to dev_ops - but thanks for asking
anyway. :)

Paolo