From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: [Qemu-devel] [PATCH v2] memory: use signed arithmetic Date: Fri, 05 Aug 2011 11:45:30 -0500 Message-ID: <4E3C1E2A.9030702@codemonkey.ws> References: <1312361774-29086-1-git-send-email-avi@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Jan Kiszka , qemu-devel@nongnu.org, kvm@vger.kernel.org, Richard Henderson To: Avi Kivity Return-path: Received: from mail-yx0-f174.google.com ([209.85.213.174]:49124 "EHLO mail-yx0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750855Ab1HEQpg (ORCPT ); Fri, 5 Aug 2011 12:45:36 -0400 Received: by yxj19 with SMTP id 19so1566814yxj.19 for ; Fri, 05 Aug 2011 09:45:36 -0700 (PDT) In-Reply-To: <1312361774-29086-1-git-send-email-avi@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 08/03/2011 03:56 AM, Avi Kivity wrote: > When trying to map an alias of a ram region, where the alias starts at > address A and we map it into address B, and A> B, we had an arithmetic > underflow. Because we use unsigned arithmetic, the underflow converted > into a large number which failed addrrange_intersects() tests. > > The concrete example which triggered this was cirrus vga mapping > the framebuffer at offsets 0xc0000-0xc7fff (relative to the start of > the framebuffer) into offsets 0xa0000 (relative to system addres space > start). > > With our favorite analogy of a windowing system, this is equivalent to > dragging a subwindow off the left edge of the screen, and failing to clip > it into its parent window which is on screen. > > Fix by switching to signed arithmetic. > > Signed-off-by: Avi Kivity Applied. Thanks. Regards, Anthony Liguori > --- > > v2: add comment about physical address width limitation to 63 bits > > exec.c | 2 +- > memory.c | 23 ++++++++++++++--------- > 2 files changed, 15 insertions(+), 10 deletions(-) > > diff --git a/exec.c b/exec.c > index 476b507..751fd89 100644 > --- a/exec.c > +++ b/exec.c > @@ -3818,7 +3818,7 @@ static void io_mem_init(void) > static void memory_map_init(void) > { > system_memory = qemu_malloc(sizeof(*system_memory)); > - memory_region_init(system_memory, "system", UINT64_MAX); > + memory_region_init(system_memory, "system", INT64_MAX); > set_system_memory_map(system_memory); > } > > diff --git a/memory.c b/memory.c > index 5f20320..be891c6 100644 > --- a/memory.c > +++ b/memory.c > @@ -22,12 +22,17 @@ unsigned memory_region_transaction_depth = 0; > > typedef struct AddrRange AddrRange; > > +/* > + * Note using signed integers limits us to physical addresses at most > + * 63 bits wide. They are needed for negative offsetting in aliases > + * (large MemoryRegion::alias_offset). > + */ > struct AddrRange { > - uint64_t start; > - uint64_t size; > + int64_t start; > + int64_t size; > }; > > -static AddrRange addrrange_make(uint64_t start, uint64_t size) > +static AddrRange addrrange_make(int64_t start, int64_t size) > { > return (AddrRange) { start, size }; > } > @@ -37,7 +42,7 @@ static bool addrrange_equal(AddrRange r1, AddrRange r2) > return r1.start == r2.start&& r1.size == r2.size; > } > > -static uint64_t addrrange_end(AddrRange r) > +static int64_t addrrange_end(AddrRange r) > { > return r.start + r.size; > } > @@ -56,9 +61,9 @@ static bool addrrange_intersects(AddrRange r1, AddrRange r2) > > static AddrRange addrrange_intersection(AddrRange r1, AddrRange r2) > { > - uint64_t start = MAX(r1.start, r2.start); > + int64_t start = MAX(r1.start, r2.start); > /* off-by-one arithmetic to prevent overflow */ > - uint64_t end = MIN(addrrange_end(r1) - 1, addrrange_end(r2) - 1); > + int64_t end = MIN(addrrange_end(r1) - 1, addrrange_end(r2) - 1); > return addrrange_make(start, end - start + 1); > } > > @@ -411,8 +416,8 @@ static void render_memory_region(FlatView *view, > MemoryRegion *subregion; > unsigned i; > target_phys_addr_t offset_in_region; > - uint64_t remain; > - uint64_t now; > + int64_t remain; > + int64_t now; > FlatRange fr; > AddrRange tmp; > > @@ -486,7 +491,7 @@ static FlatView generate_memory_topology(MemoryRegion *mr) > > flatview_init(&view); > > - render_memory_region(&view, mr, 0, addrrange_make(0, UINT64_MAX)); > + render_memory_region(&view, mr, 0, addrrange_make(0, INT64_MAX)); > flatview_simplify(&view); > > return view;