File 60afe616-x86-CPUID-rework-HLE-and-RTM-handling.... of Package xen.19910

Overview Repositories Revisions Requests Users Attributes Meta

File 60afe616-x86-CPUID-rework-HLE-and-RTM-handling.patch of Package xen.19910

# Commit 8fe24090d940d760145ccd5e234290be7418b175
# Date 2021-05-27 19:34:00 +0100
# Author Andrew Cooper <andrew.cooper3@citrix.com>
# Committer Andrew Cooper <andrew.cooper3@citrix.com>
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@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -1150,9 +1150,6 @@ void __init init_speculation_mitigations
          ((hw_smt_enabled && opt_smt) ||
           !boot_cpu_has(X86_FEATURE_SC_VERW_IDLE)) )
     {
-        setup_clear_cpu_cap(X86_FEATURE_HLE);
-        setup_clear_cpu_cap(X86_FEATURE_RTM);
-
         opt_tsx = 0;
         tsx_init();
     }
--- 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 )
--- 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) /
 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 */

Places

File 60afe616-x86-CPUID-rework-HLE-and-RTM-handling.patch of Package xen.19910

Places