Re: [PATCHv7 00/14] mm, x86/cc: Implement support for unaccepted memory

Kirill A. Shutemov
2022-07-18T17:21:58Z

On Tue, Jun 28, 2022 at 07:17:00PM +0200, Ard Biesheuvel wrote:
> On Tue, 28 Jun 2022 at 00:38, Kirill A. Shutemov
> <kirill.shutemov@linux.intel.com> wrote:
> >
> > On Mon, Jun 27, 2022 at 06:33:51PM +0200, Ard Biesheuvel wrote:
> > > > > > >
> > > > > > > Just as an idea, we can put info into UTS_VERSION which can be read from
> > > > > > > the built bzImage. We have info on SMP and preeption there already.
> > > > > > >
> > > > > >
> > > > > > Instead of hacking this into the binary, couldn't we define a protocol
> > > > > > that the kernel will call from the EFI stub (before EBS()) to identify
> > > > > > itself as an image that understands unaccepted memory, and knows how
> > > > > > to deal with it?
> > > > > >
> > > > > > That way, the firmware can accept all the memory on behalf of the OS
> > > > > > at ExitBootServices() time, unless the OS has indicated there is no
> > > > > > need to do so.
> > > > >
> > > > > I agree it would be better. But I think it would require change to EFI
> > > > > spec, no?
> > > >
> > > > Could this somehow be amended on to the UEFI Specification version 2.9
> > > > change which added all of the unaccepted memory features?
> > > >
> > >
> > > Why would this need a change in the EFI spec? Not every EFI protocol
> > > needs to be in the spec.
> >
> > My EFI knowledge is shallow. Do we do this in other cases?
> >
>
> The E in EFI means 'extensible' and the whole design of a protocol
> database using GUIDs as identifiers (which will not collide and
> therefore need no a priori coordination when defining them) is
> intended to allow extensions to be defined and implemented in a
> distributed manner.
>
> Of course, it would be fantastic if we can converge on a protocol that
> all flavors of confidential compute can use, across different OSes, so
> it is generally good if a protocol is defined in *some* shared
> specification. But this doesn't have to be the EFI spec.

I've talked with our firmware expert today and I think we have a problem
with the approach when kernel declaries support of unaccepted memory.

This apporach doesn't work if we include bootloader into the picture: if
EBS() called by bootloader we still cannot know if target kernel supports
unaccepted memory and we return to the square 1.

I think we should make it obvious from a kernel image if it supports
unaccepted memory (with UTS_VERSION or other way).

Any comments?

--
Kirill A. Shutemov