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

[Juergen Gross]

On 09/03/2015 04:52 PM, David Vrabel wrote:
> On 03/09/15 15:45, David Vrabel wrote:
> > On 03/09/15 15:38, 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).
> >
> > We expand the reserved (i.e., non-RAM) areas down so they're fully
> > covered with whole pages when we depopulate and 1:1 map them, we should
> > add extra memory regions that cover these same areas.
>
> Ignore this.  This was nonsense.
>
> We expand the reserved (i.e., non-RAM) areas so they're fully covered
> with whole pages when we depopulate and 1:1 map them, we should add the
> extra memory such that it does not overlap with with expanded regions.
> i.e., round up the start and round down the end (like Roger's patch does).

Nearly. Roger's patch rounds up start and rounds down the size. It might
add non-RAM partial pages to xen_extra_mem.


Juergen

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