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

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Message-ID: <8ff54c7b-2e64-4503-03f6-1a858d9d5746@redhat.com>
Date:	Mon, 4 Jul 2016 09:48:52 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Xiao Guangrong <guangrong.xiao@...ux.intel.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 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.

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.

Paolo