From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: x86/spec-ctrl: Allow the RDRAND/RDSEED features to be hidden

RDRAND/RDSEED can be hidden using cpuid= to mitigate SRBDS if microcode
isn't available.

This is part of XSA-320 / CVE-2020-0543.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Julien Grall <jgrall@amazon.com>

diff --git a/docs/misc/xen-command-line.pandoc b/docs/misc/xen-command-line.pandoc
index 337fbf0492..7897da55ca 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -481,12 +481,18 @@ choice of `dom0-kernel` is deprecated and not supported by all Dom0 kernels.
 This option allows for fine tuning of the facilities Xen will use, after
 accounting for hardware capabilities as enumerated via CPUID.
 
+Unless otherwise noted, options only have any effect in their negative form,
+to hide the named feature(s).  Ignoring a feature using this mechanism will
+cause Xen not to use the feature, nor offer them as usable to guests.
+
 Currently accepted:
 
 The Speculation Control hardware features `srbds-ctrl`, `md-clear`, `ibrsb`,
 `stibp`, `ibpb`, `l1d-flush` and `ssbd` are used by default if available and
-applicable.  They can be ignored, e.g. `no-ibrsb`, at which point Xen won't
-use them itself, and won't offer them to guests.
+applicable.  They can all be ignored.
+
+`rdrand` and `rdseed` can be ignored, as a mitigation to XSA-320 /
+CVE-2020-0543.
 
 ### cpuid_mask_cpu
 > `= fam_0f_rev_[cdefg] | fam_10_rev_[bc] | fam_11_rev_b`
diff --git a/xen/arch/x86/cpuid.c b/xen/arch/x86/cpuid.c
index 22d8c71a95..d07567c901 100644
--- a/xen/arch/x86/cpuid.c
+++ b/xen/arch/x86/cpuid.c
@@ -64,6 +64,16 @@ static int __init parse_xen_cpuid(const char *s)
             if ( !val )
                 setup_clear_cpu_cap(X86_FEATURE_SRBDS_CTRL);
         }
+        else if ( (val = parse_boolean("rdrand", s, ss)) >= 0 )
+        {
+            if ( !val )
+                setup_clear_cpu_cap(X86_FEATURE_RDRAND);
+        }
+        else if ( (val = parse_boolean("rdseed", s, ss)) >= 0 )
+        {
+            if ( !val )
+                setup_clear_cpu_cap(X86_FEATURE_RDSEED);
+        }
         else
             rc = -EINVAL;