[xen staging] x86/spec-ctrl: Update docs with SRBDS workaround

[patchbot@xxxxxxx]

commit 7028534d8482d25860c4d1aa8e45f0b911abfc5a
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Wed Jun 10 18:57:00 2020 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Thu Jun 11 16:26:06 2020 +0100

    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@xxxxxxxxxx>
    Acked-by: Julien Grall <jgrall@xxxxxxxxxx>
---
 docs/misc/xen-command-line.pandoc | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/docs/misc/xen-command-line.pandoc 
b/docs/misc/xen-command-line.pandoc
index 1a69c60139..fde749c669 100644
--- 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`
--
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