Re: [RFC PATCH] plugins: force slow path when plugins instrument memory ops

["Alex Bennée" <alex.bennee@linaro.org>]

Richard Henderson <richard.henderson@linaro.org> writes:

> On 4/19/23 17:12, Alex Bennée wrote:
>> The lack of SVE memory instrumentation has been an omission in plugin
>> handling since it was introduced. Fortunately we can utilise the
>> probe_* functions to force all all memory access to follow the slow
>> path. We do this by checking the access type and presence of plugin
>> memory callbacks and if set return the TLB_MMIO flag.
>> We have to jump through a few hoops in user mode to re-use the flag
>> but it was the desired effect:
>>   ./qemu-system-aarch64 -display none -serial mon:stdio \
>>     -M virt -cpu max -semihosting-config enable=on \
>>     -kernel ./tests/tcg/aarch64-softmmu/memory-sve \
>>     -plugin ./contrib/plugins/libexeclog.so,ifilter=st1w,afilter=0x40001808 -d plugin
>> gives (disas doesn't currently understand st1w):
>>    0, 0x40001808, 0xe54342a0, ".byte 0xa0, 0x42, 0x43, 0xe5", store,
>> 0x40213010, RAM, store, 0x40213014, RAM, store, 0x40213018, RAM
>> And for user-mode:
>>    ./qemu-aarch64 \
>>      -plugin contrib/plugins/libexeclog.so,afilter=0x4007c0 \
>>      -d plugin \
>>      ./tests/tcg/aarch64-linux-user/sha512-sve
>> gives:
>>    1..10
>>    ok 1 - do_test(&tests[i])
>>    0, 0x4007c0, 0xa4004b80, ".byte 0x80, 0x4b, 0x00, 0xa4", load, 0x5500800370, load, 0x5500800371, load, 0x5500800372, load, 0x5500800373, load, 0x5500800374, load, 0x5500800375, load, 0x5500800376, load, 0x5500800377, load, 0x5500800378, load, 0x5500800379, load, 0x550080037a, load, 0x550080037b, load, 0x550080037c, load, 0x550080037d, load, 0x550080037e, load, 0x550080037f, load, 0x5500800380, load, 0x5500800381, load, 0x5500800382, load, 0x5500800383, load, 0x5500800384, load, 0x5500800385, load, 0x5500800386, lo
>>    ad, 0x5500800387, load, 0x5500800388, load, 0x5500800389, load, 0x550080038a, load, 0x550080038b, load, 0x550080038c, load, 0x550080038d, load, 0x550080038e, load, 0x550080038f, load, 0x5500800390, load, 0x5500800391, load, 0x5500800392, load, 0x5500800393, load, 0x5500800394, load, 0x5500800395, load, 0x5500800396, load, 0x5500800397, load, 0x5500800398, load, 0x5500800399, load, 0x550080039a, load, 0x550080039b, load, 0x550080039c, load, 0x550080039d, load, 0x550080039e, load, 0x550080039f, load, 0x55008003a0, load, 0x55008003a1, load, 0x55008003a2, load, 0x55008003a3, load, 0x55008003a4, load, 0x55008003a5, load, 0x55008003a6, load, 0x55008003a7, load, 0x55008003a8, load, 0x55008003a9, load, 0x55008003aa, load, 0x55008003ab, load, 0x55008003ac, load, 0x55008003ad, load, 0x55008003ae, load, 0x55008003af
>> (4007c0 is the ld1b in the sha512-sve)
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>> Cc: Robert Henry <robhenry@microsoft.com>
>> Cc: Aaron Lindsay <aaron@os.amperecomputing.com>
>> ---
>>   include/exec/cpu-all.h            |  2 +-
>>   include/hw/core/cpu.h             | 17 +++++++++++++++++
>>   accel/tcg/cputlb.c                |  6 +++++-
>>   accel/tcg/user-exec.c             |  6 +++++-
>>   target/arm/tcg/sve_helper.c       |  4 ----
>>   tests/tcg/aarch64/Makefile.target |  8 ++++++++
>>   6 files changed, 36 insertions(+), 7 deletions(-)
>
> Looks good, mostly.
>
>> @@ -1530,6 +1530,7 @@ static int probe_access_internal(CPUArchState *env, target_ulong addr,
>>       target_ulong tlb_addr, page_addr;
>>       size_t elt_ofs;
>>       int flags;
>> +    bool not_fetch = true;
>>         switch (access_type) {
>>       case MMU_DATA_LOAD:
>> @@ -1540,6 +1541,7 @@ static int probe_access_internal(CPUArchState *env, target_ulong addr,
>>           break;
>>       case MMU_INST_FETCH:
>>           elt_ofs = offsetof(CPUTLBEntry, addr_code);
>> +        not_fetch = false;
>>           break;
>>       default:
>>           g_assert_not_reached();
>> @@ -1578,7 +1580,9 @@ static int probe_access_internal(CPUArchState *env, target_ulong addr,
>>       *pfull = &env_tlb(env)->d[mmu_idx].fulltlb[index];
>>         /* Fold all "mmio-like" bits into TLB_MMIO.  This is not
>> RAM.  */
>> -    if (unlikely(flags & ~(TLB_WATCHPOINT | TLB_NOTDIRTY))) {
>> +    if (unlikely(flags & ~(TLB_WATCHPOINT | TLB_NOTDIRTY))
>> +        ||
>> +        (not_fetch && cpu_plugin_mem_cbs_enabled(env_cpu(env)))) {
>
> Rather than introduce a new variable, just test access_type !=
> MMU_INST_FETCH.

w.r.t to not instrumenting the TLB accesses how ugly would something
like this be:

--8<---------------cut here---------------start------------->8---
modified   include/hw/core/cpu.h
@@ -80,10 +80,24 @@ DECLARE_CLASS_CHECKERS(CPUClass, CPU,
     typedef struct ArchCPU CpuInstanceType; \
     OBJECT_DECLARE_TYPE(ArchCPU, CpuClassType, CPU_MODULE_OBJ_NAME);
 
+/**
+ * typedef MMUAccessType - describe the type of access for cputlb
+ *
+ * When handling the access to memory we need to know the type of
+ * access we are doing. Loads and store rely on read and write page
+ * permissions where as the instruction fetch relies on execute
+ * permissions. Additional bits are used for TLB access so we can
+ * suppress instrumentation of memory when the CPU is probing.
+ */
 typedef enum MMUAccessType {
     MMU_DATA_LOAD  = 0,
     MMU_DATA_STORE = 1,
-    MMU_INST_FETCH = 2
+    MMU_INST_FETCH = 2,
+    /* MMU Mask */
+    MMU_VALID_MASK = (MMU_DATA_LOAD | MMU_DATA_STORE | MMU_INST_FETCH),
+    /* Represents the CPU walking the page table */
+    MMU_TLB_ACCESS = 0x4,
+    MMU_TLB_LOAD = MMU_DATA_LOAD | MMU_TLB_ACCESS
 } MMUAccessType;
 
 typedef struct CPUWatchpoint CPUWatchpoint;
modified   accel/tcg/cputlb.c
@@ -1503,11 +1503,12 @@ static void notdirty_write(CPUState *cpu, vaddr mem_vaddr, unsigned size,
 }
 
 static int probe_access_internal(CPUArchState *env, target_ulong addr,
-                                 int fault_size, MMUAccessType access_type,
+                                 int fault_size, MMUAccessType full_access_type,
                                  int mmu_idx, bool nonfault,
                                  void **phost, CPUTLBEntryFull **pfull,
                                  uintptr_t retaddr)
 {
+    MMUAccessType access_type = full_access_type & MMU_VALID_MASK;
     uintptr_t index = tlb_index(env, mmu_idx, addr);
     CPUTLBEntry *entry = tlb_entry(env, mmu_idx, addr);
     target_ulong tlb_addr = tlb_read_idx(entry, access_type);
@@ -1546,7 +1547,9 @@ static int probe_access_internal(CPUArchState *env, target_ulong addr,
     /* Fold all "mmio-like" bits into TLB_MMIO.  This is not RAM.  */
     if (unlikely(flags & ~(TLB_WATCHPOINT | TLB_NOTDIRTY))
         ||
-        (access_type != MMU_INST_FETCH && cpu_plugin_mem_cbs_enabled(env_cpu(env)))) {
+        (access_type != MMU_INST_FETCH &&
+         !(full_access_type & MMU_TLB_ACCESS) &&
+         cpu_plugin_mem_cbs_enabled(env_cpu(env)))) {
         *phost = NULL;
         return TLB_MMIO;
     }
--8<---------------cut here---------------end--------------->8---

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro