This particular variant has an error that causes RDSEED to always return
0xffffffff, while RDRAND works correctly.
Inspired by Linux commit 5b937a1ed64ebeba8876e398110a5790ad77407c
("x86/rdrand: Disable RDSEED on AMD Cyan Skillfish").
Like for RDRAND, permit a command line override to be used to keep
RDSEED enabled.
Signed-off-by: Jan Beulich <jbeul...@suse.com>
---
Considering how it is described, I didn't think probing RDSEED (like we
do for RDRAND) would be necessary.
Am I going too far in also updating cpuidmask_defaults here, or is us
not doing so for the RDRAND disabling actually an oversight?
Using warning_add() may not be quite appropriate, as we don't really
mean the admin to possibly override this with "cpuid=rdseed" (that's
only a last resort, in case the issue is yet more limited in scope). But
mere printk() would feel like hiding the information in the middle of
lots of other output.
--- a/xen/arch/x86/cpu-policy.c
+++ b/xen/arch/x86/cpu-policy.c
@@ -123,6 +123,10 @@ static void __init cf_check _parse_xen_c
else if ( feat == X86_FEATURE_RDRAND &&
(cpuid_ecx(1) & cpufeat_mask(X86_FEATURE_RDRAND)) )
setup_force_cpu_cap(X86_FEATURE_RDRAND);
+ else if ( feat == X86_FEATURE_RDSEED &&
+ cpuid_eax(0) >= 7 &&
+ (cpuid_count_ebx(7, 0) & cpufeat_mask(X86_FEATURE_RDSEED)) )
+ setup_force_cpu_cap(X86_FEATURE_RDSEED);
}
static int __init cf_check parse_xen_cpuid(const char *s)
--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -1220,6 +1220,24 @@ static void cf_check init_amd(struct cpu
}
break;
+ case 0x17:
+ /*
+ * Fam17 model 47 stepping 0 has an error that causes RDSEED to
+ * always return 0xffffffff (while RDRAND works correctly).
+ */
+ if (c == &boot_cpu_data &&
+ c->family == 0x47 && c->stepping == 0 &&
+ cpu_has(c, X86_FEATURE_RDSEED) &&
+ !is_forced_cpu_cap(X86_FEATURE_RDSEED)) {
+ static const char __initconst text[] =
+ "RDSEED is unreliable on this hardware;
disabling its exposure\n";
+
+ setup_clear_cpu_cap(X86_FEATURE_RDSEED);
+ cpuidmask_defaults._7ab0 &=
~cpufeat_mask(X86_FEATURE_RDSEED);
+ warning_add(text);
+ }
+ break;
+
case 0x19:
/*
* Zen3 (Fam19h model < 0x10) parts are not susceptible to
