[xen staging-4.13] x86/spec-ctrl: Update docs with SRBDS workaround

[patchbot@xxxxxxx]

commit d8e1053bfa2726d122109d137fc6c489948b1c36
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 16:28:08 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>
    (cherry picked from commit 7028534d8482d25860c4d1aa8e45f0b911abfc5a)
---
 docs/misc/xen-command-line.pandoc | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/docs/misc/xen-command-line.pandoc 
b/docs/misc/xen-command-line.pandoc
index c780312531..81e12d053c 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`
--
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