x86/cpu: populate CPUID 0x1.edx features early for self-snoop detection

Otherwise the check for the SS feature in
check_memory_type_self_snoop_errata() fails unconditionally, which leads to
X86_FEATURE_XEN_SELFSNOOP never being set.

We could also avoid this by not doing the reset_cpuinfo() for the BSP in
identify_cpu(), because SS detection uses boot_cpu_data.  However that
creates an imbalance on the state of the BSP versus the APs in the
identify_cpu() code.

I've opted for the less controversial solution of populating FEATURESET_1d
in generic_identify(), as the value is already there.  The same is done for
the AMD faulting probe code.  While there also populate the FEATURESET_1c
element, as the value is in context.

Fixes: f2663ca2e5 ("x86/cpu/intel: Clear cache self-snoop capability in CPUs with known errata")
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/cpu/common.c

@@ -490,6 +490,13 @@ static void generic_identify(struct cpuinfo_x86 *c)
 	c->apicid = phys_pkg_id((ebx >> 24) & 0xFF, 0);
 	c->phys_proc_id = c->apicid;
 
+	/*
+	 * Early init of Self Snoop support requires 0x1.edx, while there also
+	 * set 0x1.ecx as the value is in context.
+	 */
+	c->x86_capability[FEATURESET_1c] = ecx;
+	c->x86_capability[FEATURESET_1d] = edx;
+
 	eax = cpuid_eax(0x80000000);
 	if ((eax >> 16) == 0x8000)
 		c->extended_cpuid_level = eax;