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

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

On Mon, Feb 03, 2020 at 03:07:24PM +0000, Wei Liu wrote:
> 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.

Hm, I see. I'm not sure this is correct, I think the e820 entry
should look like:

addr = s;
size = PAGE_SIZE - 1;

As ranges on the e820 are inclusive, so if size ends up being
PAGE_SIZE then the entry would expand across two pages.

Anyway, this needs fixing elsewhere, and is out of the scope of this
patch.

Thanks, Roger.

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