linux-kernel - Re: [PATCH 0/2] KVM: MMU: support VMAs that got remap_pfn

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Date:	Mon, 4 Jul 2016 15:37:35 +0800
From:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
To:	Neo Jia <cjia@...dia.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, Kirti Wankhede <kwankhede@...dia.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH 0/2] KVM: MMU: support VMAs that got remap_pfn_range-ed



On 07/04/2016 03:03 PM, Neo Jia wrote:
> On Mon, Jul 04, 2016 at 02:39:22PM +0800, Xiao Guangrong wrote:
>>
>>
>> On 06/30/2016 09:01 PM, Paolo Bonzini wrote:
>>> The vGPU folks would like to trap the first access to a BAR by setting
>>> vm_ops on the VMAs produced by mmap-ing a VFIO device.  The fault handler
>>> then can use remap_pfn_range to place some non-reserved pages in the VMA.
>>
>> Why does it require fetching the pfn when the fault is triggered rather
>> than when mmap() is called?
>
> Hi Guangrong,
>
> as such mapping information between virtual mmio to physical mmio is only available
> at runtime.

Sorry, i do not know what the different between mmap() and the time VM actually
accesses the memory for your case. Could you please more detail?

>
>>
>> Why the memory mapped by this mmap() is not a portion of MMIO from
>> underlayer physical device? If it is a valid system memory, is this interface
>> really needed to implemented in vfio? (you at least need to set VM_MIXEDMAP
>> if it mixed system memory with MMIO)
>>
>
> It actually is a portion of the physical mmio which is set by vfio mmap.

So i do not think we need to care its refcount, i,e, we can consider it as reserved_pfn,
Paolo?

>
>> IIUC, the kernel assumes that VM_PFNMAP is a continuous memory, e.g, like
>> current KVM and vaddr_get_pfn() in vfio, but it seems nvdia's patchset
>> breaks this semantic as ops->validate_map_request() can adjust the physical
>> address arbitrarily. (again, the name 'validate' should be changed to match
>> the thing as it is really doing)
>
> The vgpu api will allow you to adjust the target mmio address and the size via
> validate_map_request, but it is still physical contiguous as <start_pfn, size>.

Okay, the interface confused us, maybe this interface need to be cooked to reflect
to this fact.

Thanks!