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[35.185.214.157]) by smtp.gmail.com with ESMTPSA id v11-20020a1709028d8b00b00165105518f6sm7390006plo.287.2022.06.14.08.43.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 14 Jun 2022 08:43:52 -0700 (PDT) Date: Tue, 14 Jun 2022 15:43:48 +0000 From: Sean Christopherson To: Michael Roth Cc: x86@kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, linux-efi@vger.kernel.org, platform-driver-x86@vger.kernel.org, linux-coco@lists.linux.dev, linux-mm@kvack.org, Thomas Gleixner , Ingo Molnar , Joerg Roedel , Tom Lendacky , "H. Peter Anvin" , Ard Biesheuvel , Paolo Bonzini , Vitaly Kuznetsov , Jim Mattson , Andy Lutomirski , Dave Hansen , Sergio Lopez , Peter Gonda , Peter Zijlstra , Srinivas Pandruvada , David Rientjes , Dov Murik , Tobin Feldman-Fitzthum , Borislav Petkov , Vlastimil Babka , "Kirill A . Shutemov" , Andi Kleen , "Dr . David Alan Gilbert" , brijesh.ksingh@gmail.com, tony.luck@intel.com, marcorr@google.com, sathyanarayanan.kuppuswamy@linux.intel.com Subject: Re: [PATCH v12 19/46] x86/kernel: Make the .bss..decrypted section shared in RMP table Message-ID: References: <20220307213356.2797205-1-brijesh.singh@amd.com> <20220307213356.2797205-20-brijesh.singh@amd.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > > --- > > 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. > 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.