Re: [Xen-devel] [PATCH v6 05/11] x86/hyperv: setup hypercall page

[Wei Liu <wl@xxxxxxx>]

On Mon, Feb 03, 2020 at 04:01:54PM +0100, Roger Pau Monné wrote:
> On Fri, Jan 31, 2020 at 05:49:24PM +0000, Wei Liu wrote:
> > Hyper-V uses a technique called overlay page for its hypercall page. It
> > will insert a backing page to the guest when the hypercall functionality
> > is enabled. That means we can use a page that is not backed by real
> > memory for hypercall page.
> > 
> > Use the top-most addressable page for that purpose. Adjust e820 map
> > accordingly.
> 
> Can you add this is done to avoid page shattering and to make sure
> Xen isn't overwriting any MMIO area which might be present at lower
> addresses?

NP.

> 
> > +
> > +static void __init e820_fixup(struct e820map *e820)
> > +{
> > +    uint64_t s = HV_HCALL_MFN << PAGE_SHIFT;
> > +
> > +    if ( !e820_add_range(e820, s, s + PAGE_SIZE, E820_RESERVED) )
> 
> I think end should be s + PAGE_SIZE - 1, or else it expands across two
> pages?

No, it shouldn't.

E820 entry records the size of the region, which is calculated as
end-start. The one usage in pv/shim.c follows the same pattern here.

Wei.

> 
> Thanks, Roger.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel