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

[Alex Bennée]

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