Re: [PATCH v3] x86/boot/sev: Avoid shared GHCB page for early memory acceptance

[Ard Biesheuvel]

On Thu, 17 Apr 2025 at 18:08, Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
>
> On 4/11/25 14:00, Ard Biesheuvel wrote:
> > On Fri, 11 Apr 2025 at 20:40, Borislav Petkov <bp@xxxxxxxxx> wrote:
> >>
> >> On Thu, Apr 10, 2025 at 03:28:51PM +0200, Ard Biesheuvel wrote:
> >>> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
> >>>
> >>> Communicating with the hypervisor using the shared GHCB page requires
> >>> clearing the C bit in the mapping of that page. When executing in the
> >>> context of the EFI boot services, the page tables are owned by the
> >>> firmware, and this manipulation is not possible.
> >>>
> >>> So switch to a different API for accepting memory in SEV-SNP guests, one
> >>
> >> That being the GHCB MSR protocol, it seems.
> >>
> >
> > Yes.
> >
> >> And since Tom co-developed, I guess we wanna do that.
> >>
> >> But then how much slower do we become?
> >>
> >
> > Non-EFI stub boot will become slower if the memory that is used to
> > decompress the kernel has not been accepted yet. But given how heavily
> > SEV-SNP depends on EFI boot, this typically only happens on kexec, as
> > that is the only boot path that goes through the traditional
> > decompressor.
>
> Some quick testing showed no significant differences in kexec booting
> and testing shows everything seems to be good.
>

Thanks.

> But, in testing with non-2M sized memory (e.g. a guest with 4097M of
> memory) and without the change to how SNP is detected before
> sev_enable() is called, we hit the error path in arch_accept_memory() in
> arch/x86/boot/compressed/mem.c and the boot crashes.
>

Right. So this is because sev_snp_enabled() is based on sev_status,
which has not been set yet at this point, right?

And for the record, could you please indicate whether you are ok with
the co-developed-by/signed-off-by credits on this patch (and
subsequent revisions)?