Re: [PATCH v2 5/6] efi: xen: Implement memory descriptor lookup based on hypercall

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Mon, Oct 03, 2022 at 05:59:52PM +0200, Ard Biesheuvel wrote:
> On Mon, 3 Oct 2022 at 17:29, Demi Marie Obenour
> <demi@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Mon, Oct 03, 2022 at 01:26:24PM +0200, Ard Biesheuvel wrote:
> > > +     out_md->phys_addr       = info->mem.addr;
> >
> > This will be equal to phys_addr, not the actual start of the memory
> > region.
> >
> > > +     out_md->num_pages       = info->mem.size >> EFI_PAGE_SHIFT;
> >
> > Similarly, this will be the number of bytes in the memory region
> > after phys_addr, not the total number of bytes in the region.  These two
> > differences mean that this function is not strictly equivalent to the
> > original efi_mem_desc_lookup().
> >
> > I am not sure if this matters in practice, but I thought you would want
> > to be aware of it.
> 
> This is a bit disappointing. Is there no way to obtain this
> information via a Xen hypercall?

I don't think so, unfortunately. That said, with the below adjustment, I
think that's okay for the _current_ users of efi_mem_desc_lookup().

> In any case, it means we'll need to round down phys_addr to page size
> at the very least.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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