Messages in this thread | | | From | Dexuan Cui <> | Subject | Does earlyprintk=ttyS0 work for an AMD SNP guest on KVM? | Date | Thu, 16 Feb 2023 04:40:14 +0000 |
| |
Hi all, With the earlyprintk=ttyS0 kernel parameter, a C-bit mode Linux SNP guest on Hyper-V always decides to crash via sev_es_terminate() in do_boot_stage2_vc(), because early_setup_ghcb() fails:
early_setup_ghcb() -> set_page_decrypted() -> set_clr_page_flags() -> split_large_pmd() -> alloc_pgt_page() fails to allocate memory.
static void *alloc_pgt_page(void *context) { ... /* Validate there is space available for a new page. */ if (pages->pgt_buf_offset >= pages->pgt_buf_size) { ... return NULL; } ... }
alloc_pgt_page() fails to allocate memory because both pages->pgt_buf_offset and pages->pgt_buf_size are zero.
pgt_data.pgt_buf_size is zero because of this line in initialize_identity_maps() pgt_data.pgt_buf_size = BOOT_PGT_SIZE - BOOT_INIT_PGT_SIZE;
void initialize_identity_maps(void *rmode) { ... top_level_pgt = read_cr3_pa(); if (p4d_offset((pgd_t *)top_level_pgt, 0) == (p4d_t *)_pgtable) { pgt_data.pgt_buf = _pgtable + BOOT_INIT_PGT_SIZE; pgt_data.pgt_buf_size = BOOT_PGT_SIZE - BOOT_INIT_PGT_SIZE; memset(pgt_data.pgt_buf, 0, pgt_data.pgt_buf_size); } else { pgt_data.pgt_buf = _pgtable; pgt_data.pgt_buf_size = BOOT_PGT_SIZE; memset(pgt_data.pgt_buf, 0, pgt_data.pgt_buf_size); top_level_pgt = (unsigned long)alloc_pgt_page(&pgt_data); }
In arch/x86/include/asm/boot.h, BOOT_PGT_SIZE equals BOOT_INIT_PGT_SIZE if CONFIG_RANDOMIZE_BASE is not defined (which is my case): # define BOOT_INIT_PGT_SIZE (6*4096)
# ifdef CONFIG_RANDOMIZE_BASE ... # ifdef CONFIG_X86_VERBOSE_BOOTUP # define BOOT_PGT_SIZE (19*4096) # else /* !CONFIG_X86_VERBOSE_BOOTUP */ # define BOOT_PGT_SIZE (17*4096) # endif # else /* !CONFIG_RANDOMIZE_BASE */ # define BOOT_PGT_SIZE BOOT_INIT_PGT_SIZE # endif
I think this means: if CONFIG_RANDOMIZE_BASE is not defined, earlyprintk=ttyS0 also doesn't work for an SNP guest on KVM? Sorry I don't have a KVM environment at hand to test it.
If I define CONFIG_RANDOMIZE_BASE, my C-bit mode SNP guest crashes even ealier -- it looks like CONFIG_RANDOMIZE_BASE is incompatible with my guest on Hyper-V due to some reason I don't know.
Do you always use CONFIG_RANDOMIZE_BASE for a SNP guest on KVM and does earlyprintk=ttyS0 work for you?
Can you please share your thoughts? Thanks!
Thanks, -- Dexuan
| |