linux-kernel - Does earlyprintk=ttyS0 work for an AMD SNP guest on KVM?

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Message-ID: <SA1PR21MB13359DBABC5625368E369A42BFA09@SA1PR21MB1335.namprd21.prod.outlook.com>
Date:   Thu, 16 Feb 2023 04:40:14 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Tom Lendacky <thomas.lendacky@....com>,
        Borislav Petkov <bp@...en8.de>,
        "sandipan.das@....com" <sandipan.das@....com>,
        "Gupta, Pankaj" <pankaj.gupta@....com>,
        "ray.huang@....com" <ray.huang@....com>,
        "brijesh.singh@....com" <brijesh.singh@....com>,
        "michael.roth@....com" <michael.roth@....com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
CC:     Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Does earlyprintk=ttyS0 work for an AMD SNP guest on KVM?

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