Re: [Xen-devel] [PATCH 4/8] x86/hyperv: setup hypercall page

[Wei Liu <wl@xxxxxxx>]

On Sun, Dec 29, 2019 at 07:54:30PM +0000, Michael Kelley wrote:
[...]
> > 
> > +static void __init setup_hypercall_page(void)
> > +{
> > +    union hv_x64_msr_hypercall_contents hypercall_msr;
> > +
> > +    /* Unfortunately there isn't a really good way to unwind Xen to
> > +     * not use Hyper-V hooks, so panic if anything goes wrong.
> > +     *
> > +     * In practice if page allocation fails this early on it is
> > +     * unlikely we can get a working system later.
> > +     */
> > +    hv_hypercall_page = alloc_domheap_page(NULL, 0);
> > +    if ( !hv_hypercall_page )
> > +        panic("Failed to allocate Hyper-V hypercall page\n");
> > +
> > +    hv_hypercall = __map_domain_page_global(hv_hypercall_page);
> > +    if ( !hv_hypercall )
> > +        panic("Failed to map Hyper-V hypercall page\n");
> > +
> > +    rdmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
> > +    hypercall_msr.enable = 1;
> > +    hypercall_msr.guest_physical_address = 
> > page_to_maddr(hv_hypercall_page);
> 
> The "guest_physical_address" field is actually the guest physical page number.
> So the physical address needs to be right shifted 12 bits before being stored
> here.  I'd recommend using HV_HYP_PAGE_SHIFT from hyperv-tlfs.h as
> the shift value; it was introduced to deal with the possibility that the page
> size used and expected by the Hyper-V interface is different from the page
> size used by the guest VM (which can happen on ARM64, though not on x86).

Good catch, and thanks for the tip here.

I will fix this in the next version.

Wei.

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