Re: [PATCH Part1 v5 30/38] x86/compressed/64: store Confidential Computing blob address in bootparams

Borislav Petkov
2021-08-27T14:14:44Z

On Fri, Aug 20, 2021 at 10:19:25AM -0500, Brijesh Singh wrote:
> From: Michael Roth <michael.roth@amd.com>
>
> When the Confidential Computing blob is located by the boot/compressed
> kernel, store a pointer to it in bootparams->cc_blob_address to avoid
> the need for the run-time kernel to rescan the EFI config table to find
> it again.
>
> Since this function is also shared by the run-time kernel, this patch

Here's "this patch" again... but you know what to do.

> also adds the logic to make use of bootparams->cc_blob_address when it
> has been initialized.
>
> Signed-off-by: Michael Roth <michael.roth@amd.com>
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
> arch/x86/kernel/sev-shared.c | 40 ++++++++++++++++++++++++++----------
> 1 file changed, 29 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
> index 651980ddbd65..6f70ba293c5e 100644
> --- a/arch/x86/kernel/sev-shared.c
> +++ b/arch/x86/kernel/sev-shared.c
> @@ -868,7 +868,6 @@ static enum es_result vc_handle_rdtsc(struct ghcb *ghcb,
> return ES_OK;
> }
>
> -#ifdef BOOT_COMPRESSED
> static struct setup_data *get_cc_setup_data(struct boot_params *bp)
> {
> struct setup_data *hdr = (struct setup_data *)bp->hdr.setup_data;
> @@ -888,6 +887,16 @@ static struct setup_data *get_cc_setup_data(struct boot_params *bp)
> * 1) Search for CC blob in the following order/precedence:
> * - via linux boot protocol / setup_data entry
> * - via EFI configuration table
> + * 2) If found, initialize boot_params->cc_blob_address to point to the
> + * blob so that uncompressed kernel can easily access it during very
> + * early boot without the need to re-parse EFI config table
> + * 3) Return a pointer to the CC blob, NULL otherwise.
> + *
> + * For run-time/uncompressed kernel:
> + *
> + * 1) Search for CC blob in the following order/precedence:
> + * - via linux boot protocol / setup_data entry

Why would you do this again if the boot/compressed kernel has already
searched for it?

> + * - via boot_params->cc_blob_address

Yes, that is the only thing you need to do in the runtime kernel - see
if cc_blob_address is not 0. And all the work has been done by the
decompressor kernel already.

> * 2) Return a pointer to the CC blob, NULL otherwise.
> */
> static struct cc_blob_sev_info *sev_snp_probe_cc_blob(struct boot_params *bp)
> @@ -897,9 +906,11 @@ static struct cc_blob_sev_info *sev_snp_probe_cc_blob(struct boot_params *bp)
> struct setup_data header;
> u32 cc_blob_address;
> } *sd;
> +#ifdef __BOOT_COMPRESSED
> unsigned long conf_table_pa;
> unsigned int conf_table_len;
> bool efi_64;
> +#endif

That function turns into an unreadable mess with that #ifdef
__BOOT_COMPRESSED slapped everywhere.

It seems the cleanest thing to do is to do what we do with
acpi_rsdp_addr: do all the parsing in boot/compressed/ and pass it on
through boot_params. Kernel proper simply reads the pointer.

Which means, you can stick all that cc_blob figuring out functionality
in arch/x86/boot/compressed/sev.c instead.

Thx.

--
Regards/Gruss,
Boris.