x86/spec-ctrl: Update docs with SRBDS workaround

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 1a69c60139d3c14d7ccbf826eaadfd668c65cbb9..fde749c66981f5cb3603ada2668f08069d0b5d15 100644 (file)
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -502,16 +502,21 @@ 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` can be used to override the default disabling of the feature on certain
-AMD systems.  Its negative form can of course also be used to suppress use and
-exposure of the feature.
+`rdrand` and `rdseed` can be ignored, as a mitigation to XSA-320 /
+CVE-2020-0543.  The RDRAND feature is disabled by default on certain AMD
+systems, due to possible malfunctions after ACPI S3 suspend/resume.  `rdrand`
+may be used in its positive form to override Xen's default behaviour on these
+systems, and make the feature fully usable.
 
 ### cpuid_mask_cpu
 > `= fam_0f_rev_[cdefg] | fam_10_rev_[bc] | fam_11_rev_b`