RE: Does earlyprintk=ttyS0 work for an AMD SNP guest on KVM?

From: Dexuan Cui <decui@microsoft.com>
To: Jeremi Piotrowski <jpiotrowski@linux.microsoft.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>,
	Borislav Petkov <bp@alien8.de>,
	"sandipan.das@amd.com" <sandipan.das@amd.com>,
	"Gupta, Pankaj" <pankaj.gupta@amd.com>,
	"ray.huang@amd.com" <ray.huang@amd.com>,
	"brijesh.singh@amd.com" <brijesh.singh@amd.com>,
	"michael.roth@amd.com" <michael.roth@amd.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"x86@kernel.org" <x86@kernel.org>,
	Tianyu Lan <Tianyu.Lan@microsoft.com>,
	"linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: Does earlyprintk=ttyS0 work for an AMD SNP guest on KVM?
Date: Thu, 16 Feb 2023 17:58:49 +0000	[thread overview]
Message-ID: <SA1PR21MB13358707043E2901958819AEBFA09@SA1PR21MB1335.namprd21.prod.outlook.com> (raw)
In-Reply-To: <20230216091431.GA10166@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>

> From: Jeremi Piotrowski <jpiotrowski@linux.microsoft.com>
> Sent: Thursday, February 16, 2023 1:15 AM
> > ...
> > 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);
> 
> I just tested an SNP guest on KVM with and without
> CONFIG_RANDOMIZE_BASE.
> In both cases we end up in the else() branch.
> With CONFIG_RANDOMIZE_BASE BOOT_PGT_SIZE=0x13000
> Without CONFIG_RANDOMMIZE_BASE BOOT_PGT_SIZE=0x6000.
> 
> So in both cases pgt_data.pgt_buf_size != 0.
> 
> Getting into that first branch would require having 5-level paging supported
> (CONFIG_X86_5LEVEL=y) and enabled inside the guest, I don't have that on
> any
> hardware I have access to.
> 
> Jeremi

CONFIG_X86_5LEVEL is not set for my kernel.

The comment before the first branch says:
  On 4-level paging, p4d_offset(top_level_pgt, 0) is equal to 'top_level_pgt'.

IIUC this means 'top_level_pgt' is equal to '_pgtable'? i.e. without 
CONFIG_RANDOMIZE_BASE, pgt_data.pgt_buf_size should be 0.

Not sure why it's not getting into the first branch for you.