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

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Message-ID: <CAMj1kXFVeygkdnhtkGHsKfRgGXoMRpdjQ-wTO1RBLU_wERs_=Q@mail.gmail.com>
Date: Thu, 17 Apr 2025 18:38:07 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Tom Lendacky <thomas.lendacky@....com>
Cc: Borislav Petkov <bp@...en8.de>, Ard Biesheuvel <ardb+git@...gle.com>, linux-efi@...r.kernel.org, 
	x86@...nel.org, mingo@...nel.org, linux-kernel@...r.kernel.org, 
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, 
	Dionna Amalie Glaze <dionnaglaze@...gle.com>, Kevin Loughlin <kevinloughlin@...gle.com>
Subject: Re: [PATCH v3] x86/boot/sev: Avoid shared GHCB page for early memory acceptance

On Thu, 17 Apr 2025 at 18:21, Tom Lendacky <thomas.lendacky@....com> wrote:
>
> On 4/17/25 11:14, Ard Biesheuvel wrote:
> > On Thu, 17 Apr 2025 at 18:08, Tom Lendacky <thomas.lendacky@....com> wrote:
> >>
> >> On 4/11/25 14:00, Ard Biesheuvel wrote:
> >>> On Fri, 11 Apr 2025 at 20:40, Borislav Petkov <bp@...en8.de> wrote:
> >>>>
> >>>> On Thu, Apr 10, 2025 at 03:28:51PM +0200, Ard Biesheuvel wrote:
> >>>>> From: Ard Biesheuvel <ardb@...nel.org>
> >>>>>
> >>>>> 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)

--- a/arch/x86/boot/compressed/mem.c
+++ b/arch/x86/boot/compressed/mem.c
@@ -34,11 +34,14 @@ static bool early_is_tdx_guest(void)

 void arch_accept_memory(phys_addr_t start, phys_addr_t end)
 {
+       static bool sevsnp;
+
        /* Platform-specific memory-acceptance call goes here */
        if (early_is_tdx_guest()) {
                if (!tdx_accept_memory(start, end))
                        panic("TDX: Failed to accept memory\n");
-       } else if (sev_snp_enabled()) {
+       } else if (sevsnp || (sev_get_status() & MSR_AMD64_SEV_SNP_ENABLED)) {
+               sevsnp = true;
                snp_accept_memory(start, end);
        } else {
                error("Cannot accept memory: unknown platform\n");

> >
> > 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)?
>
> Yep, I'm fine with that.
>

Thanks.