[xen stable-4.13] x86/cpuid: Rework HLE and RTM handling

[patchbot@xxxxxxx]

commit f17d848c4caa4fa562f3121c430667b6df838700
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Fri Jun 4 15:04:56 2021 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Jun 4 15:04:56 2021 +0200

    x86/cpuid: Rework HLE and RTM handling
    
    The TAA mitigation offered the option to hide the HLE and RTM CPUID bits,
    which has caused some migration compatibility problems.
    
    These two bits are special.  Annotate them with ! to emphasise this point.
    
    Hardware Lock Elision (HLE) may or may not be visible in CPUID, but is
    disabled in microcode on all CPUs, and has been removed from the 
architecture.
    Do not advertise it to VMs by default.
    
    Restricted Transactional Memory (RTM) may or may not be visible in CPUID, 
and
    may or may not be configured in force-abort mode.  Have tsx_init() note
    whether RTM has been configured into force-abort mode, so
    guest_common_feature_adjustments() can conditionally hide it from VMs by
    default.
    
    The host policy values for HLE/RTM may or may not be set, depending on any
    previous running kernel's choice of visibility, and Xen's choice.  TSX is
    available on any CPU which enumerates a TSX-hiding mechanism, so instead of
    doing a two-step to clobber any hiding, scan CPUID, then set the visibility,
    just force visibility of the bits in the first place.
    
    With the HLE/RTM bits now unilaterally visible in the host policy,
    xc_cpuid_apply_policy() can construct a more appropriate policy out of thin
    air for pre-4.13 VMs with no CPUID data in their migration stream, and
    specifically one where HLE/RTM doesn't potentially disappear behind the back
    of a running VM.
    
    Fixes: 8c4330818f6 ("x86/spec-ctrl: Mitigate the TSX Asynchronous Abort 
sidechannel")
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    master commit: 8fe24090d940d760145ccd5e234290be7418b175
    master date: 2021-05-27 19:34:00 +0100
---
 xen/arch/x86/tsx.c                          | 24 ++++++++++++++++++++++++
 xen/include/public/arch-x86/cpufeatureset.h |  4 ++--
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/tsx.c b/xen/arch/x86/tsx.c
index 2d202a0d4e..a5d5944fe8 100644
--- a/xen/arch/x86/tsx.c
+++ b/xen/arch/x86/tsx.c
@@ -44,6 +44,30 @@ void tsx_init(void)
             rdmsrl(MSR_ARCH_CAPABILITIES, caps);
 
         cpu_has_tsx_ctrl = !!(caps & ARCH_CAPS_TSX_CTRL);
+
+        /*
+         * The TSX features (HLE/RTM) are handled specially.  They both
+         * enumerate features but, on certain parts, have mechanisms to be
+         * hidden without disrupting running software.
+         *
+         * At the moment, we're running in an unknown context (WRT hiding -
+         * particularly if another fully fledged kernel ran before us) and
+         * depending on user settings, may elect to continue hiding them from
+         * native CPUID instructions.
+         *
+         * Xen doesn't use TSX itself, but use cpu_has_{hle,rtm} for various
+         * system reasons, mostly errata detection, so the meaning is more
+         * useful as "TSX infrastructure available", as opposed to "features
+         * advertised and working".
+         *
+         * Force the features to be visible in Xen's view if we see any of the
+         * infrastructure capable of hiding them.
+         */
+        if ( cpu_has_tsx_ctrl )
+        {
+            setup_force_cpu_cap(X86_FEATURE_HLE);
+            setup_force_cpu_cap(X86_FEATURE_RTM);
+        }
     }
 
     if ( cpu_has_tsx_ctrl )
diff --git a/xen/include/public/arch-x86/cpufeatureset.h 
b/xen/include/public/arch-x86/cpufeatureset.h
index 3162ee0378..a1d1619643 100644
--- a/xen/include/public/arch-x86/cpufeatureset.h
+++ b/xen/include/public/arch-x86/cpufeatureset.h
@@ -195,14 +195,14 @@ XEN_CPUFEATURE(FSGSBASE,      5*32+ 0) /*A  
{RD,WR}{FS,GS}BASE instructions */
 XEN_CPUFEATURE(TSC_ADJUST,    5*32+ 1) /*S  TSC_ADJUST MSR available */
 XEN_CPUFEATURE(SGX,           5*32+ 2) /*   Software Guard extensions */
 XEN_CPUFEATURE(BMI1,          5*32+ 3) /*A  1st bit manipulation extensions */
-XEN_CPUFEATURE(HLE,           5*32+ 4) /*A  Hardware Lock Elision */
+XEN_CPUFEATURE(HLE,           5*32+ 4) /*!A Hardware Lock Elision */
 XEN_CPUFEATURE(AVX2,          5*32+ 5) /*A  AVX2 instructions */
 XEN_CPUFEATURE(FDP_EXCP_ONLY, 5*32+ 6) /*!  x87 FDP only updated on exception. 
*/
 XEN_CPUFEATURE(SMEP,          5*32+ 7) /*S  Supervisor Mode Execution 
Protection */
 XEN_CPUFEATURE(BMI2,          5*32+ 8) /*A  2nd bit manipulation extensions */
 XEN_CPUFEATURE(ERMS,          5*32+ 9) /*A  Enhanced REP MOVSB/STOSB */
 XEN_CPUFEATURE(INVPCID,       5*32+10) /*H  Invalidate Process Context ID */
-XEN_CPUFEATURE(RTM,           5*32+11) /*A  Restricted Transactional Memory */
+XEN_CPUFEATURE(RTM,           5*32+11) /*!A Restricted Transactional Memory */
 XEN_CPUFEATURE(PQM,           5*32+12) /*   Platform QoS Monitoring */
 XEN_CPUFEATURE(NO_FPU_SEL,    5*32+13) /*!  FPU CS/DS stored as zero */
 XEN_CPUFEATURE(MPX,           5*32+14) /*S  Memory Protection Extensions */
--
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