linux-kernel - Re: [PATCH v8 33/40] x86/compressed/64: add identity mapping for Confidential Computing blob

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Message-ID: <14a7c745-b6c3-8d9c-144f-2d06b80159e6@intel.com>
Date:   Fri, 10 Dec 2021 11:52:28 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Brijesh Singh <brijesh.singh@....com>, 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
Cc:     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>,
        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>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v8 33/40] x86/compressed/64: add identity mapping for
 Confidential Computing blob

On 12/10/21 7:43 AM, Brijesh Singh wrote:
> +static void sev_prep_identity_maps(void)
> +{
> +	/*
> +	 * The ConfidentialComputing blob is used very early in uncompressed
> +	 * kernel to find the in-memory cpuid table to handle cpuid
> +	 * instructions. Make sure an identity-mapping exists so it can be
> +	 * accessed after switchover.
> +	 */
> +	if (sev_snp_enabled()) {
> +		struct cc_blob_sev_info *cc_info =
> +			(void *)(unsigned long)boot_params->cc_blob_address;
> +
> +		add_identity_map((unsigned long)cc_info,
> +				 (unsigned long)cc_info + sizeof(*cc_info));
> +		add_identity_map((unsigned long)cc_info->cpuid_phys,
> +				 (unsigned long)cc_info->cpuid_phys + cc_info->cpuid_len);
> +	}

The casting here is pretty ugly.  Also, isn't ->cpuid_phys already a
u64?  Whats the idea behind casting it?

I also have a sneaking suspicion that a single "unsigned long cc_blob"
could remove virtually all the casting.  Does this work?

	unsigned long cc_blob = boot_params->cc_blob_addres;
	struct cc_blob_sev_info *cc_info;

	add_identity_map(cc_blob, cc_blob + sizeof(*cc_info));

	cc_info = (struct cc_blob_sev_info *)cc_blob;
	add_identity_map(cc_info->cpuid_phys,
			 cc_info->cpuid_phys + cc_info->cpuid_len);