Re: [PATCH v7 6/6] x86/efi: Safely enable unaccepted memory in UEFI

Kirill A. Shutemov
2023-04-04T20:25:03Z

On Tue, Apr 04, 2023 at 09:49:52PM +0200, Ard Biesheuvel wrote:
> On Tue, 4 Apr 2023 at 20:09, Kirill A. Shutemov <kirill@shutemov.name> wrote:
> >
> > On Tue, Apr 04, 2023 at 10:57:52AM -0700, Dave Hansen wrote:
> > > On 4/4/23 10:45, Kirill A. Shutemov wrote:
> > > > I still think it is a bad idea.
> > > >
> > > > As I asked before, please include my
> > > >
> > > > Nacked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > > >
> > > > into the patch.
> > >
> > > I was pretty opposed to this when I first saw it too. But, Tom and
> > > company have worn down my opposition a bit.
> > >
> > > The fact is that we have upstream kernels out there with SEV-SNP support
> > > that don't know anything about unaccepted memory. They're either
> > > relegated to using the pre-accepted memory (4GB??) or _some_ entity
> > > needs to accept the memory. That entity obviously can't be the kernel
> > > unless we backport unaccepted memory support.
> > >
> > > This both lets the BIOS be the page-accepting entity _and_ allows the
> > > entity to delegate that to the kernel when it needs to.
> > >
> > > As much as I want to nak this and pretend that that those existing
> > > kernel's don't exist, my powers of self-delusion do have their limits.
> > >
> > > If our AMD friends don't do this, what is their alternative?
> >
> > The alternative is coordination on the host side: VMM can load a BIOS that
> > pre-accepts all memory if the kernel is older.
> >
>
> And how does one identify such a kernel? How does the VMM know which
> kernel the guest is going to load after it boots?

VMM has to know what it is running. Yes, it is cumbersome. But enabling
phase for a feature is often rough. It will get smoother overtime.

> > I know that it is not convenient for VMM, but it is technically possible.
> >
> > Introduce an ABI with an expiration date is much more ugly. And nobody
> > will care about the expiration date, until you will try to remove it.
> >
>
> None of us are thrilled about this, but the simple reality is that
> there are kernels that do not understand unaccepted memory.

How is it different from any other feature the kernel is not [yet] aware
of?

Like if we boot a legacy kernel on machine with persistent memory or
memory attached over CLX, it will not see it as conventional memory.

> EFI being
> an extensible, generic, protocol based programmatic interface, the
> best way of informing the loader that a kernel does understand it is
> /not/ by adding some flag to some highly arch and OS specific header,
> but to discover a protocol and call it.
>
> We're past arguing that a legitimate need exists for a solution to
> this problem. So what solution are you proposing?

I described the solution multiple times. You just don't like it.

--
Kiryl Shutsemau / Kirill A. Shutemov