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

[Ard Biesheuvel]

On Thu, 17 Apr 2025 at 22:01, Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
>
> On 4/17/25 12:26, Tom Lendacky wrote:
> > On 4/17/25 11:38, Ard Biesheuvel wrote:
> >> On Thu, 17 Apr 2025 at 18:21, Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
> >>>
> >>> On 4/17/25 11:14, Ard Biesheuvel wrote:
> >>>> 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?
> >>>
> >>> Correct.
> >>>
> >>
> >> OK. Would this do the trick? (with asm/sev.h added to the #includes)
> >
> > Yes, that works for booting. Let me do some kexec testing and get back
> > to you. Sorry, that might not be until tomorrow, though.
>
> Ok, found some time... looks good with kexec, too.
>

Thanks!