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

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Date:	Mon, 4 Jul 2016 18:35:55 -0700
From:	Neo Jia <cjia@...dia.com>
To:	Xiao Guangrong <guangrong.xiao@...ux.intel.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 Tue, Jul 05, 2016 at 09:19:40AM +0800, Xiao Guangrong wrote:
> 
> 
> On 07/04/2016 11:33 PM, Neo Jia wrote:
> 
> >>>
> >>>Sorry, I think I misread the "allocation" as "mapping". We only delay the
> >>>cpu mapping, not the allocation.
> >>
> >>So how to understand your statement:
> >>"at that moment nobody has any knowledge about how the physical mmio gets virtualized"
> >>
> >>The resource, physical MMIO region, has been allocated, why we do not know the physical
> >>address mapped to the VM?
> >>
> >
> >>From a device driver point of view, the physical mmio region never gets allocated until
> >the corresponding resource is requested by clients and granted by the mediated device driver.
> 
> Hmm... but you told me that you did not delay the allocation. :(

Hi Guangrong,

The allocation here is the allocation of device resource, and the only way to
access that kind of device resource is via a mmio region of some pages there.

For example, if VM needs resource A, and the only way to access resource A is
via some kind of device memory at mmio address X.

So, we never defer the allocation request during runtime, we just setup the
CPU mapping later when it actually gets accessed.

> 
> So it returns to my original question: why not allocate the physical mmio region in mmap()?
> 

Without running anything inside the VM, how do you know how the hw resource gets
allocated, therefore no knowledge of the use of mmio region.

Thanks,
Neo

> 
> 
> 
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