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

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Date:   Mon, 13 Dec 2021 11:54:47 -0600
From:   Michael Roth <michael.roth@....com>
To:     Dave Hansen <dave.hansen@...el.com>
CC:     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>,
        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>,
        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 Fri, Dec 10, 2021 at 11:52:28AM -0800, Dave Hansen wrote:
> 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);

Yes, the cc->cpuid_phys cast is not needed, and your suggested implementation
is clearer and compiles/runs without any issues. I'll implement it this way for
the next spin. Thanks!