Re: [PATCH v12 19/46] x86/kernel: Make the .bss..decrypted section shared in RMP table

Sean Christopherson
2022-06-14T16:15:44Z

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On Tue, Jun 14, 2022, Tom Lendacky wrote:
> On 6/14/22 10:43, Sean Christopherson wrote:
> > On Tue, Jun 14, 2022, Sean Christopherson wrote:
> > > s/Brijesh/Michael
> > >
> > > On Mon, Mar 07, 2022, Brijesh Singh wrote:
> > > > The encryption attribute for the .bss..decrypted section is cleared in the
> > > > initial page table build. This is because the section contains the data
> > > > that need to be shared between the guest and the hypervisor.
> > > >
> > > > When SEV-SNP is active, just clearing the encryption attribute in the
> > > > page table is not enough. The page state need to be updated in the RMP
> > > > table.
> > > >
> > > > Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> > > > ---
> > > > arch/x86/kernel/head64.c | 13 +++++++++++++
> > > > 1 file changed, 13 insertions(+)
> > > >
> > > > diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
> > > > index 83514b9827e6..656d2f3e2cf0 100644
> > > > --- a/arch/x86/kernel/head64.c
> > > > +++ b/arch/x86/kernel/head64.c
> > > > @@ -143,7 +143,20 @@ static unsigned long __head sme_postprocess_startup(struct boot_params *bp, pmdv
> > > > if (sme_get_me_mask()) {
> > > > vaddr = (unsigned long)__start_bss_decrypted;
> > > > vaddr_end = (unsigned long)__end_bss_decrypted;
> > > > +
> > > > for (; vaddr < vaddr_end; vaddr += PMD_SIZE) {
> > > > + /*
> > > > + * On SNP, transition the page to shared in the RMP table so that
> > > > + * it is consistent with the page table attribute change.
> > > > + *
> > > > + * __start_bss_decrypted has a virtual address in the high range
> > > > + * mapping (kernel .text). PVALIDATE, by way of
> > > > + * early_snp_set_memory_shared(), requires a valid virtual
> > > > + * address but the kernel is currently running off of the identity
> > > > + * mapping so use __pa() to get a *currently* valid virtual address.
> > > > + */
> > > > + early_snp_set_memory_shared(__pa(vaddr), __pa(vaddr), PTRS_PER_PMD);
> > >
> > > This breaks SME on Rome and Milan when compiling with clang-13. I haven't been
> > > able to figure out exactly what goes wrong. printk isn't functional at this point,
> > > and interactive debug during boot on our test systems is beyond me. I can't even
> > > verify that the bug is specific to clang because the draconian build system for our
> > > test systems apparently is stuck pointing at gcc-4.9.
> > >
> > > I suspect the issue is related to relocation and/or encrypting memory, as skipping
> > > the call to early_snp_set_memory_shared() if SNP isn't active masks the issue.
> > > I've dug through the assembly and haven't spotted a smoking gun, e.g. no obvious
> > > use of absolute addresses.
> > >
> > > Forcing a VM through the same path doesn't fail. I can't test an SEV guest at the
> > > moment because INIT_EX is also broken.
> >
> > The SEV INIT_EX was a PEBKAC issue. An SEV guest boots just fine with a clang-built
> > kernel, so either it's a finnicky relocation issue or something specific to SME.
>
> I just built and booted 5.19-rc2 with clang-13 and SME enabled without issue:
>
> [ 4.118226] Memory Encryption Features active: AMD SME

Phooey.

> Maybe something with your kernel config? Can you send me your config?

Attached. If you can't repro, I'll find someone on our end to work on this.

Thanks!

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