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:

quoted

+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!