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

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Date:   Fri, 27 Aug 2021 16:15:14 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH Part1 v5 30/38] x86/compressed/64: store Confidential
 Computing blob address in bootparams

On Fri, Aug 20, 2021 at 10:19:25AM -0500, Brijesh Singh wrote:
> From: Michael Roth <michael.roth@....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@....com>
> Signed-off-by: Brijesh Singh <brijesh.singh@....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.

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