linux-kernel - Re: [PATCH] kvm, x86: Properly check whether a pfn is an MMIO or not

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Date:	Wed, 22 Jun 2016 14:11:30 +0000
From:	"Raslan, KarimAllah" <karahmed@...zon.de>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H . Peter Anvin" <hpa@...or.com>,
	Radim Krčmář <rkrcmar@...hat.com>,
	Borislav Petkov <bp@...e.de>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Toshi Kani <toshi.kani@....com>,
	Tony Luck <tony.luck@...el.com>
Subject: Re: [PATCH] kvm, x86: Properly check whether a pfn is an MMIO or not



On 6/22/16, 3:41 PM, "Paolo Bonzini" <pbonzini@...hat.com> wrote:

>
>
>On 22/06/2016 04:34, KarimAllah Ahmed wrote:
>> pfn_valid check is not sufficient because it only checks if a page has a struct
>> page or not, if for example "mem=" was passed to the kernel some valid pages
>> won't have a struct page. This means that if guests were assigned valid memory
>> that lies after the mem= boundary it will be passed uncached to the guest no
>> matter what the guest caching attributes are for this memory.
>
>How can you pass memory after the mem= boundary to the guest?

Memory regions can be assigned to the guest as long as you've a valid host
virtual address. You can always get a host virtual address for any RAM that
lives after 'mem=' for example by simply mmaping /dev/mem or even using some
special kernel module to mmap these regions from user space.

>
>Paolo
>