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

Tom Lendacky
2022-06-14T16:02:06Z

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.