Re: [Xen-devel] [PATCH for-4.12 1/8] dom0/pvh: align allocation and mapping order to start address

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Jan 30, 2019 at 12:37:28PM +0000, Wei Liu wrote:
> On Wed, Jan 30, 2019 at 11:36:39AM +0100, Roger Pau Monne wrote:
> > Due to the recent changes in the iommu mapping logic, the start
> > addresses provided need to be aligned to the order intended to be
> > mapped.
> > 
> 
> Can you reference some commits here? What would happen if the address is
> not aligned?

See:
https://lists.xenproject.org/archives/html/xen-devel/2019-01/msg01030.html

and

https://lists.xenproject.org/archives/html/xen-devel/2019-01/msg01503.html

> > dom0 PVH domain builder didn't take this into account when populating
> > the p2m, fix this by making sure the order is chosen so that the start
> > address is aligned to it.
> > 
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > ---
> > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> > ---
> > Without this patch trying to create a PVH dom0 will trigger an assert
> > on certain hardware depending on the memory map.
> > ---
> >  xen/arch/x86/hvm/dom0_build.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/xen/arch/x86/hvm/dom0_build.c b/xen/arch/x86/hvm/dom0_build.c
> > index 51cf490811..a571d15c13 100644
> > --- a/xen/arch/x86/hvm/dom0_build.c
> > +++ b/xen/arch/x86/hvm/dom0_build.c
> > @@ -152,6 +152,8 @@ static int __init pvh_populate_memory_range(struct 
> > domain *d,
> >  
> >          order = get_order_from_pages(end - start + 1);
> >          order = min(order ? order - 1 : 0, max_order);
> > +        /* The order allocated and populated must be aligned to the 
> > address. */
> > +        order = min(order, start ? find_first_set_bit(start) : MAX_ORDER);
> 
> Isn't max_order better here?

It will yield the same result because order has already been limited
by max_order. I've used MAX_ORDER directly because it's a constant and
could be faster than loading the value in max_order. You could also
use 'order' instead of MAX_ORDER and will also yield the same result.

Thanks, Roger.

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