[xen staging] x86/cpu-policy: Allow for levelling of VERW side effects

[patchbot@xxxxxxx]

commit de17162cafd27f2865a3102a2ec0f386a02ed03d
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Thu Feb 29 11:26:40 2024 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri Mar 1 20:14:19 2024 +0000

    x86/cpu-policy: Allow for levelling of VERW side effects
    
    MD_CLEAR and FB_CLEAR need OR-ing across a migrate pool.  Allow this, by
    having them unconditinally set in max, with the host values reflected in
    default.  Annotate the bits as having special properies.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/cpu-policy.c                   | 24 ++++++++++++++++++++++++
 xen/arch/x86/include/asm/cpufeature.h       |  1 +
 xen/include/public/arch-x86/cpufeatureset.h |  4 ++--
 3 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/cpu-policy.c b/xen/arch/x86/cpu-policy.c
index 609db6946f..2c6f03057b 100644
--- a/xen/arch/x86/cpu-policy.c
+++ b/xen/arch/x86/cpu-policy.c
@@ -442,6 +442,16 @@ static void __init 
guest_common_max_feature_adjustments(uint32_t *fs)
         __set_bit(X86_FEATURE_RSBA, fs);
         __set_bit(X86_FEATURE_RRSBA, fs);
 
+        /*
+         * These bits indicate that the VERW instruction may have gained
+         * scrubbing side effects.  With pooling, they mean "you might migrate
+         * somewhere where scrubbing is necessary", and may need exposing on
+         * unaffected hardware.  This is fine, because the VERW instruction
+         * has been around since the 286.
+         */
+        __set_bit(X86_FEATURE_MD_CLEAR, fs);
+        __set_bit(X86_FEATURE_FB_CLEAR, fs);
+
         /*
          * The Gather Data Sampling microcode mitigation (August 2023) has an
          * adverse performance impact on the CLWB instruction on SKX/CLX/CPX.
@@ -486,6 +496,20 @@ static void __init 
guest_common_default_feature_adjustments(uint32_t *fs)
              cpu_has_rdrand && !is_forced_cpu_cap(X86_FEATURE_RDRAND) )
             __clear_bit(X86_FEATURE_RDRAND, fs);
 
+        /*
+         * These bits indicate that the VERW instruction may have gained
+         * scrubbing side effects.  The max policy has them set for migration
+         * reasons, so reset the default policy back to the host values in
+         * case we're unaffected.
+         */
+        __clear_bit(X86_FEATURE_MD_CLEAR, fs);
+        if ( cpu_has_md_clear )
+            __set_bit(X86_FEATURE_MD_CLEAR, fs);
+
+        __clear_bit(X86_FEATURE_FB_CLEAR, fs);
+        if ( cpu_has_fb_clear )
+            __set_bit(X86_FEATURE_FB_CLEAR, fs);
+
         /*
          * The Gather Data Sampling microcode mitigation (August 2023) has an
          * adverse performance impact on the CLWB instruction on SKX/CLX/CPX.
diff --git a/xen/arch/x86/include/asm/cpufeature.h 
b/xen/arch/x86/include/asm/cpufeature.h
index be16492c68..ad24d0fa88 100644
--- a/xen/arch/x86/include/asm/cpufeature.h
+++ b/xen/arch/x86/include/asm/cpufeature.h
@@ -178,6 +178,7 @@ static inline bool boot_cpu_has(unsigned int feat)
 #define cpu_has_avx512_4fmaps   boot_cpu_has(X86_FEATURE_AVX512_4FMAPS)
 #define cpu_has_avx512_vp2intersect 
boot_cpu_has(X86_FEATURE_AVX512_VP2INTERSECT)
 #define cpu_has_srbds_ctrl      boot_cpu_has(X86_FEATURE_SRBDS_CTRL)
+#define cpu_has_md_clear        boot_cpu_has(X86_FEATURE_MD_CLEAR)
 #define cpu_has_rtm_always_abort boot_cpu_has(X86_FEATURE_RTM_ALWAYS_ABORT)
 #define cpu_has_tsx_force_abort boot_cpu_has(X86_FEATURE_TSX_FORCE_ABORT)
 #define cpu_has_serialize       boot_cpu_has(X86_FEATURE_SERIALIZE)
diff --git a/xen/include/public/arch-x86/cpufeatureset.h 
b/xen/include/public/arch-x86/cpufeatureset.h
index b230d3a690..0374cec3a2 100644
--- a/xen/include/public/arch-x86/cpufeatureset.h
+++ b/xen/include/public/arch-x86/cpufeatureset.h
@@ -262,7 +262,7 @@ XEN_CPUFEATURE(AVX512_4FMAPS, 9*32+ 3) /*A  AVX512 Multiply 
Accumulation Single
 XEN_CPUFEATURE(FSRM,          9*32+ 4) /*A  Fast Short REP MOVS */
 XEN_CPUFEATURE(AVX512_VP2INTERSECT, 9*32+8) /*a  VP2INTERSECT{D,Q} insns */
 XEN_CPUFEATURE(SRBDS_CTRL,    9*32+ 9) /*   MSR_MCU_OPT_CTRL and 
RNGDS_MITG_DIS. */
-XEN_CPUFEATURE(MD_CLEAR,      9*32+10) /*A  VERW clears microarchitectural 
buffers */
+XEN_CPUFEATURE(MD_CLEAR,      9*32+10) /*!A VERW clears microarchitectural 
buffers */
 XEN_CPUFEATURE(RTM_ALWAYS_ABORT, 9*32+11) /*! June 2021 TSX defeaturing in 
microcode. */
 XEN_CPUFEATURE(TSX_FORCE_ABORT, 9*32+13) /* MSR_TSX_FORCE_ABORT.RTM_ABORT */
 XEN_CPUFEATURE(SERIALIZE,     9*32+14) /*A  SERIALIZE insn */
@@ -334,7 +334,7 @@ XEN_CPUFEATURE(DOITM,              16*32+12) /*   Data 
Operand Invariant Timing
 XEN_CPUFEATURE(SBDR_SSDP_NO,       16*32+13) /*A  No Shared Buffer Data Read 
or Sideband Stale Data Propagation */
 XEN_CPUFEATURE(FBSDP_NO,           16*32+14) /*A  No Fill Buffer Stale Data 
Propagation */
 XEN_CPUFEATURE(PSDP_NO,            16*32+15) /*A  No Primary Stale Data 
Propagation */
-XEN_CPUFEATURE(FB_CLEAR,           16*32+17) /*A  Fill Buffers cleared by VERW 
*/
+XEN_CPUFEATURE(FB_CLEAR,           16*32+17) /*!A Fill Buffers cleared by VERW 
*/
 XEN_CPUFEATURE(FB_CLEAR_CTRL,      16*32+18) /*   
MSR_OPT_CPU_CTRL.FB_CLEAR_DIS */
 XEN_CPUFEATURE(RRSBA,              16*32+19) /*!  Restricted RSB Alternative */
 XEN_CPUFEATURE(BHI_NO,             16*32+20) /*A  No Branch History Injection  
*/
--
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