Re: [PATCH v2] x86/kexec: Add EFI config table identity mapping for kexec kernel

[Baoquan He]

On 07/07/23 at 11:38am, Tao Liu wrote:
> Hi Borislav,
> 
> Thanks for the patch review!
> 
> On Thu, Jul 6, 2023 at 1:34 AM Borislav Petkov <bp@xxxxxxxxx> wrote:
> >
> > On Thu, Jun 01, 2023 at 03:20:44PM +0800, Tao Liu wrote:
> > > A kexec kernel bootup hang is observed on Intel Atom cpu due to unmapped
> >
> > s/cpu/CPU/g
> >
> > > EFI config table.
> > >
> > > Currently EFI system table is identity-mapped for the kexec kernel, but EFI
> > > config table is not mapped explicitly:
> >
> > Why does the EFI config table *need* to be mapped explicitly?
> >
> > >     commit 6bbeb276b71f ("x86/kexec: Add the EFI system tables and ACPI
> > >                           tables to the ident map")
> > >
> > > Later in the following 2 commits, EFI config table will be accessed when
> > > enabling sev at kernel startup.
> >
> > What does SEV have to do with an Intel problem?
> 
> For the 2 questions above. The call stack is follows:
> 
> head_64.S:.Lon_kernel_cs(which is before extract_kernel) -> sev_enable
> -> snp_init -> find_cc_blob -> find_cc_blob_efi. No matter what cpu,
> with CONFIG_AMD_MEM_ENCRYPT enabled, all will enter sev_enable() and
> go through these functions. I assume it is harmless for Intel cpu,

I am wondering why we don't detect the cpu type and return early inside
sev_enable() if it's Intel cpu.

We can't rely on CONFIG_AMD_MEM_ENCRYPT to decide if the code need be
executed or not because we usually enable them all in distros.

Thanks
Baoquan