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