x86/amd: Work around CLFLUSH ordering on older parts

On pre-CLFLUSHOPT AMD CPUs, CLFLUSH is weakely ordered with everything,
including reads and writes to the address, and LFENCE/SFENCE instructions.

This creates a multitude of problematic corner cases, laid out in the manual.
Arrange to use MFENCE on both sides of the CLFLUSH to force proper ordering.

This is part of XSA-402.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 062868a5a8b428b85db589fa9a6d6e43969ffeb9
master date: 2022-06-09 14:23:07 +0200

diff --git a/xen/arch/x86/cpu/amd.c b/xen/arch/x86/cpu/amd.c
index 2ef59e22dc312c043a3231e5e20e6850a4323c11..142f34af5f70892ee7c7c642d8ee9ef35db9ec9a 100644 (file)
--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -787,6 +787,14 @@ static void init_amd(struct cpuinfo_x86 *c)
        if (!cpu_has_lfence_dispatch)
                __set_bit(X86_FEATURE_MFENCE_RDTSC, c->x86_capability);
 
+       /*
+        * On pre-CLFLUSHOPT AMD CPUs, CLFLUSH is weakly ordered with
+        * everything, including reads and writes to address, and
+        * LFENCE/SFENCE instructions.
+        */
+       if (!cpu_has_clflushopt)
+               setup_force_cpu_cap(X86_BUG_CLFLUSH_MFENCE);
+
        switch(c->x86)
        {
        case 0xf ... 0x11:

diff --git a/xen/arch/x86/flushtlb.c b/xen/arch/x86/flushtlb.c
index 0c912b8669f87c16b3aba3b9df83a42c01fcb73e..dcbb4064012e05f0dc483e79a16e56fecc2b7e10 100644 (file)
--- a/xen/arch/x86/flushtlb.c
+++ b/xen/arch/x86/flushtlb.c
@@ -259,6 +259,13 @@ unsigned int flush_area_local(const void *va, unsigned int flags)
     return flags;
 }
 
+/*
+ * On pre-CLFLUSHOPT AMD CPUs, CLFLUSH is weakly ordered with everything,
+ * including reads and writes to address, and LFENCE/SFENCE instructions.
+ *
+ * This function only works safely after alternatives have run.  Luckily, at
+ * the time of writing, we don't flush the caches that early.
+ */
 void cache_flush(const void *addr, unsigned int size)
 {
     /*
@@ -268,6 +275,8 @@ void cache_flush(const void *addr, unsigned int size)
     unsigned int clflush_size = current_cpu_data.x86_clflush_size ?: 16;
     const void *end = addr + size;
 
+    alternative("", "mfence", X86_BUG_CLFLUSH_MFENCE);
+
     addr -= (unsigned long)addr & (clflush_size - 1);
     for ( ; addr < end; addr += clflush_size )
     {
@@ -283,7 +292,9 @@ void cache_flush(const void *addr, unsigned int size)
                        [p] "m" (*(const char *)(addr)));
     }
 
-    alternative("", "sfence", X86_FEATURE_CLFLUSHOPT);
+    alternative_2("",
+                  "sfence", X86_FEATURE_CLFLUSHOPT,
+                  "mfence", X86_BUG_CLFLUSH_MFENCE);
 }
 
 void cache_writeback(const void *addr, unsigned int size)

diff --git a/xen/include/asm-x86/cpufeatures.h b/xen/include/asm-x86/cpufeatures.h
index fe2f97354fb6135c462067085e0cb31051118b11..09f619459bc7fc65e672fefe39792b011c7da1c9 100644 (file)
--- a/xen/include/asm-x86/cpufeatures.h
+++ b/xen/include/asm-x86/cpufeatures.h
@@ -46,6 +46,7 @@ XEN_CPUFEATURE(XEN_IBT,           X86_SYNTH(27)) /* Xen uses CET Indirect Branch
 #define X86_BUG(x) ((FSCAPINTS + X86_NR_SYNTH) * 32 + (x))
 
 #define X86_BUG_FPU_PTRS          X86_BUG( 0) /* (F)X{SAVE,RSTOR} doesn't save/restore FOP/FIP/FDP. */
+#define X86_BUG_CLFLUSH_MFENCE    X86_BUG( 2) /* MFENCE needed to serialise CLFLUSH */
 
 /* Total number of capability words, inc synth and bug words. */
 #define NCAPINTS (FSCAPINTS + X86_NR_SYNTH + X86_NR_BUG) /* N 32-bit words worth of info */