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

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Message-ID: <577A1765.6080606@linux.intel.com>
Date:	Mon, 4 Jul 2016 15:59:33 +0800
From:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
To:	Paolo Bonzini <pbonzini@...hat.com>, Neo Jia <cjia@...dia.com>
Cc:	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 03:48 PM, Paolo Bonzini wrote:
>
>
> On 04/07/2016 09:37, Xiao Guangrong wrote:
>>>>
>>>
>>> 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?
>
> nVidia provided me (offlist) with a simple patch that modified VFIO to
> exhibit the problem, and it didn't use reserved PFNs.  This is why the
> commit message for the patch is not entirely accurate.
>

It's clear now.

> But apart from this, it's much more obvious to consider the refcount.
> The x86 MMU code doesn't care if the page is reserved or not;
> mmu_set_spte does a kvm_release_pfn_clean, hence it makes sense for
> hva_to_pfn_remapped to try doing a get_page (via kvm_get_pfn) after
> invoking the fault handler, just like the get_user_pages family of
> function does.

Well,  it's little strange as you always try to get refcont
for a PFNMAP region without MIXEDMAP which indicates all the memory
in this region is no 'struct page' backend.

But it works as kvm_{get, release}_* have already been aware of
reserved_pfn, so i am okay with it......