Messages in this thread | | | Date | Tue, 14 Jun 2022 11:01:55 -0500 | Subject | Re: [PATCH v12 19/46] x86/kernel: Make the .bss..decrypted section shared in RMP table | From | Tom Lendacky <> |
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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
Maybe something with your kernel config? Can you send me your config?
Thanks, Tom
> >> The crash incurs a very, very slow reboot, and I was out of cycles to work on this >> about three hours ago. If someone on the AMD side can repro, it would be much >> appreciated.
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