linux-kernel - Re: [GIT PULL] Btrfs

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Date:	Mon, 18 Nov 2013 10:35:55 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Chris Mason <chris.mason@...ionio.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Dulshani Gunawardhana <dulshani.gunawardhana89@...il.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Mark Brown <broonie@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>
Subject: Re: [GIT PULL] Btrfs

On Sun, Nov 17, 2013 at 11:36:04AM +0200, Gleb Natapov wrote:
> On Fri, Nov 15, 2013 at 03:57:23PM +0100, Heiko Carstens wrote:
> > On Fri, Nov 15, 2013 at 02:42:08PM +0100, Geert Uytterhoeven wrote:
> > > I was just going to comment that
> > > 
> > > +       const void *zero_page = (const void *) page_to_phys(ZERO_PAGE(0));
> > > 
> > > won't fly. You can't just cast a physical address to "void *".
> > 
> > Ouch.. I think that only works on s390 because we have a 1:1 mapping for
> > physical to virtual addresses in kernel space due to our split address spaces.
> > 
> > So for btrfs and kvm it should be page_to_virt(), and for the dma_map_single()
> > case I have no idea. :)
> Can you send updated patch for kvm please?

See below. page_to_virt() is only defined for a couple of architectures, so I
used __va(page_to_phys()) instead.
I tested the patch on s390 only...

>From b19687bad7e878aaed6edb786a22c6b05e886b97 Mon Sep 17 00:00:00 2001
From: Heiko Carstens <heiko.carstens@...ibm.com>
Date: Mon, 18 Nov 2013 10:05:57 +0100
Subject: [PATCH] kvm: kvm_clear_guest_page(): fix empty_zero_page usage

Using the address of 'empty_zero_page' as source address in order to clear
a page is wrong. On some architectures empty_zero_page is only the pointer
to the struct page of the empty_zero_page.
Therefore the clear page operation would copy the contents of a couple of
struct pages instead of clearing a page.
For kvm only arm64 is affected by this bug.

To fix this use the ZERO_PAGE macro instead which will return the struct
page address of the empty_zero_page on all architectures.

Signed-off-by: Heiko Carstens <heiko.carstens@...ibm.com>
---
 virt/kvm/kvm_main.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 662f34c3287e..a0aa84b5941a 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1615,8 +1615,9 @@ EXPORT_SYMBOL_GPL(kvm_read_guest_cached);
 
 int kvm_clear_guest_page(struct kvm *kvm, gfn_t gfn, int offset, int len)
 {
-	return kvm_write_guest_page(kvm, gfn, (const void *) empty_zero_page,
-				    offset, len);
+	const void *zero_page = (const void *) __va(page_to_phys(ZERO_PAGE(0)));
+
+	return kvm_write_guest_page(kvm, gfn, zero_page, offset, len);
 }
 EXPORT_SYMBOL_GPL(kvm_clear_guest_page);
 
-- 
1.8.3.4

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