Re: [PATCH] xen/p2m: fix extra memory regions accounting

[Juergen Gross]

On 09/03/2015 04:38 PM, Roger Pau Monnà wrote:
> El 03/09/15 a les 14.25, Juergen Gross ha escrit:
> > On 09/03/2015 02:05 PM, Roger Pau Monne wrote:
> > > On systems with memory maps with ranges that don't end at page
> > > boundaries,
> > > like:
> > >
> > > [...]
> > > (XEN)  0000000000100000 - 00000000dfdf9c00 (usable)
> > > (XEN)  00000000dfdf9c00 - 00000000dfe4bc00 (ACPI NVS)
> > > [...]
> > >
> > > xen_add_extra_mem will create a protected range that ends up at
> > > 0xdfdf9c00,
> > > but the function used to check if a memory address is inside of a
> > > protected
> > > range works with pfns, which means that an attempt to map 0xdfdf9c00
> > > will be
> > > refused because the check is performed against 0xdfdf9000 instead of
> > > 0xdfdf9c00.
> > >
> > > In order to fix this, make sure that the ranges that are added to the
> > > xen_extra_mem array are aligned to page boundaries.
> > >
> > > Signed-off-by: Roger Pau Monnà <roger.pau@xxxxxxxxxx>
> > > Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > > Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> > > Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
> > > Cc: Juergen Gross <jgross@xxxxxxxx>
> > > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
> > > ---
> > > AFAICT this patch needs to be backported to 3.19, 4.0, 4.1 and 4.2.
> > > ---
> > >    arch/x86/xen/setup.c | 6 ++++++
> > >    1 file changed, 6 insertions(+)
> > >
> > > diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
> > > index 55f388e..dcf5865 100644
> > > --- a/arch/x86/xen/setup.c
> > > +++ b/arch/x86/xen/setup.c
> > > @@ -68,6 +68,9 @@ static void __init xen_add_extra_mem(phys_addr_t
> > > start, phys_addr_t size)
> > >    {
> > >        int i;
> > >
> > > +    start = PAGE_ALIGN(start);
> > > +    size &= PAGE_MASK;
> >
> > This is not correct. If start wasn't page aligned and size was, you'll
> > add one additional page to xen_extra_mem.
>
> I'm not understanding this, let's put an example:
>
> start = 0x8c00
> size = 0x1000
>
> After the fixup added above this would become:
>
> start = 0x9000
> size = 0x1000
>
> So if anything, I'm adding one page less (because 0x8000 was partly
> added, and with the fixup it is not added).

You'd add 9c00-9cff which shouldn't be added (in this example you
shouldn't add anything as no complete page is covered by the range).

You need something like:

end = (start + size) & PAGE_MASK;
start = PAGE_ALIGN(start);
size = end - start;
if (!size)
     return;


Juergen

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