Re: [Qemu-devel] [RFC PATCH] QEMU may write to system_memory before guest starts

[Yury Kotov]

19.03.2019, 12:39, "Dr. David Alan Gilbert" <address@hidden>:
> * Yury Kotov (address@hidden) wrote:
>>  This patch isn't intended to merge. Just to reproduce a problem.
>>
>>  The test for x-ignore-shread capability fails on aarch64 + tcg:
>>  Memory content inconsistency at 44c00000 first_byte = 2 last_byte = 1 
>> current = d1 hit_edge = 1
>>  Memory content inconsistency at 44c01000 first_byte = 2 last_byte = 1 
>> current = 77 hit_edge = 1
>>
>>  I expected that QEMU doesn't write to guest RAM until VM starts, but it
>>  happens in this test. By this patch I found what causes this problem.
>>
>>  Backtrace:
>>  0 0x00007fb9e40affea in __memcpy_avx_unaligned () at 
>> ../sysdeps/x86_64/multiarch/memcpy-avx-unaligned.S:118
>>  1 0x000055f4a296dd84 in address_space_write_rom_internal () at exec.c:3458
>>  2 0x000055f4a296de3a in address_space_write_rom () at exec.c:3479
>>  3 0x000055f4a2d519ff in rom_reset () at hw/core/loader.c:1101
>>  4 0x000055f4a2d475ec in qemu_devices_reset () at hw/core/reset.c:69
>>  5 0x000055f4a2c90a28 in qemu_system_reset () at vl.c:1675
>>  6 0x000055f4a2c9851d in main () at vl.c:4552
>>
>>  To fix this particular we can skip these writes for the first system_reset
>>  if -incoming is set. But I'm not sure how to fix this problem in general.
>>  May be to introduce a contract which forbids to write to system_ram in such
>>  case?
>>
>>  What do you think?
>
> I thought that ROMs would either:
>    a) Be mapped shared from a file but then read-only and unwritten
> or
>    b) Be written to during boot - but this wouldn't be main memory, so
> wouldn't be affected by your shared flag.
>
> So which case is happening here? How are the ROMs being mapped - is this
> actually a write to main memory or the RAMBlock backing just the ROM?

It writes to the main memory. Just what I wanted to catch. For separated
RAMBlocks it works fine because separated blocks aren't shared and as a result
these aren't readonly.

P.S. For x86 qtests with this patch are passed.

Regards,
Yury

>
> Dave
>
>>  Signed-off-by: Yury Kotov <address@hidden>
>>  ---
>>   backends/hostmem-file.c | 2 +-
>>   exec.c | 15 +++++++++++++--
>>   include/exec/cpu-common.h | 2 ++
>>   include/exec/memory.h | 3 +++
>>   include/qemu/mmap-alloc.h | 2 +-
>>   migration/ram.c | 2 ++
>>   util/mmap-alloc.c | 6 ++++--
>>   util/oslib-posix.c | 2 +-
>>   vl.c | 8 ++++++++
>>   9 files changed, 35 insertions(+), 7 deletions(-)
>>
>>  diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
>>  index ce54788048..146fa2bc70 100644
>>  --- a/backends/hostmem-file.c
>>  +++ b/backends/hostmem-file.c
>>  @@ -61,7 +61,7 @@ file_backend_memory_alloc(HostMemoryBackend *backend, 
>> Error **errp)
>>       memory_region_init_ram_from_file(&backend->mr, OBJECT(backend),
>>                                        name,
>>                                        backend->size, fb->align,
>>  - (backend->share ? RAM_SHARED : 0) |
>>  + (backend->share ? (RAM_SHARED | RAM_READONLY) : 0) |
>>                                        (fb->is_pmem ? RAM_PMEM : 0),
>>                                        fb->mem_path, errp);
>>       g_free(name);
>>  diff --git a/exec.c b/exec.c
>>  index 1d4f3784d6..cd86e9b837 100644
>>  --- a/exec.c
>>  +++ b/exec.c
>>  @@ -1863,7 +1863,7 @@ static void *file_ram_alloc(RAMBlock *block,
>>       }
>>
>>       area = qemu_ram_mmap(fd, memory, block->mr->align,
>>  - block->flags & RAM_SHARED);
>>  + block->flags & RAM_SHARED, block->flags & RAM_READONLY);
>>       if (area == MAP_FAILED) {
>>           error_setg_errno(errp, errno,
>>                            "unable to map backing store for guest RAM");
>>  @@ -2259,7 +2259,7 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, 
>> MemoryRegion *mr,
>>       int64_t file_size;
>>
>>       /* Just support these ram flags by now. */
>>  - assert((ram_flags & ~(RAM_SHARED | RAM_PMEM)) == 0);
>>  + assert((ram_flags & ~(RAM_SHARED | RAM_PMEM | RAM_READONLY)) == 0);
>>
>>       if (xen_enabled()) {
>>           error_setg(errp, "-mem-path not supported with Xen");
>>  @@ -2485,6 +2485,17 @@ void qemu_ram_remap(ram_addr_t addr, ram_addr_t 
>> length)
>>           }
>>       }
>>   }
>>  +
>>  +void qemu_ram_unprotect_all(void)
>>  +{
>>  + RAMBlock *block;
>>  + rcu_read_lock();
>>  + RAMBLOCK_FOREACH(block) {
>>  + int ret = mprotect(block->host, block->max_length, PROT_READ | 
>> PROT_WRITE);
>>  + assert(ret == 0);
>>  + }
>>  + rcu_read_unlock();
>>  +}
>>   #endif /* !_WIN32 */
>>
>>   /* Return a host pointer to ram allocated with qemu_ram_alloc.
>>  diff --git a/include/exec/cpu-common.h b/include/exec/cpu-common.h
>>  index cef8b88a2a..2ad875be06 100644
>>  --- a/include/exec/cpu-common.h
>>  +++ b/include/exec/cpu-common.h
>>  @@ -63,6 +63,8 @@ typedef void CPUWriteMemoryFunc(void *opaque, hwaddr 
>> addr, uint32_t value);
>>   typedef uint32_t CPUReadMemoryFunc(void *opaque, hwaddr addr);
>>
>>   void qemu_ram_remap(ram_addr_t addr, ram_addr_t length);
>>  +void qemu_ram_unprotect_all(void);
>>  +
>>   /* This should not be used by devices. */
>>   ram_addr_t qemu_ram_addr_from_host(void *ptr);
>>   RAMBlock *qemu_ram_block_by_name(const char *name);
>>  diff --git a/include/exec/memory.h b/include/exec/memory.h
>>  index 1625913f84..b1cb5a48d7 100644
>>  --- a/include/exec/memory.h
>>  +++ b/include/exec/memory.h
>>  @@ -126,6 +126,9 @@ typedef struct IOMMUNotifier IOMMUNotifier;
>>   /* RAM is a persistent kind memory */
>>   #define RAM_PMEM (1 << 5)
>>
>>  +/* RAM is readonly*/
>>  +#define RAM_READONLY (1 << 6)
>>  +
>>   static inline void iommu_notifier_init(IOMMUNotifier *n, IOMMUNotify fn,
>>                                          IOMMUNotifierFlag flags,
>>                                          hwaddr start, hwaddr end,
>>  diff --git a/include/qemu/mmap-alloc.h b/include/qemu/mmap-alloc.h
>>  index ef04f0ed5b..f704d9b7e3 100644
>>  --- a/include/qemu/mmap-alloc.h
>>  +++ b/include/qemu/mmap-alloc.h
>>  @@ -7,7 +7,7 @@ size_t qemu_fd_getpagesize(int fd);
>>
>>   size_t qemu_mempath_getpagesize(const char *mem_path);
>>
>>  -void *qemu_ram_mmap(int fd, size_t size, size_t align, bool shared);
>>  +void *qemu_ram_mmap(int fd, size_t size, size_t align, bool shared, bool 
>> readonly);
>>
>>   void qemu_ram_munmap(int fd, void *ptr, size_t size);
>>
>>  diff --git a/migration/ram.c b/migration/ram.c
>>  index 35bd6213e9..252fc80e6c 100644
>>  --- a/migration/ram.c
>>  +++ b/migration/ram.c
>>  @@ -4211,6 +4211,8 @@ static int ram_load(QEMUFile *f, void *opaque, int 
>> version_id)
>>        */
>>       rcu_read_lock();
>>
>>  + qemu_ram_unprotect_all();
>>  +
>>       if (postcopy_running) {
>>           ret = ram_load_postcopy(f);
>>       }
>>  diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
>>  index 8565885420..7dc194d909 100644
>>  --- a/util/mmap-alloc.c
>>  +++ b/util/mmap-alloc.c
>>  @@ -75,9 +75,10 @@ size_t qemu_mempath_getpagesize(const char *mem_path)
>>       return getpagesize();
>>   }
>>
>>  -void *qemu_ram_mmap(int fd, size_t size, size_t align, bool shared)
>>  +void *qemu_ram_mmap(int fd, size_t size, size_t align, bool shared, bool 
>> readonly)
>>   {
>>       int flags;
>>  + int prots;
>>       int guardfd;
>>       size_t offset;
>>       size_t pagesize;
>>  @@ -128,9 +129,10 @@ void *qemu_ram_mmap(int fd, size_t size, size_t align, 
>> bool shared)
>>       flags = MAP_FIXED;
>>       flags |= fd == -1 ? MAP_ANONYMOUS : 0;
>>       flags |= shared ? MAP_SHARED : MAP_PRIVATE;
>>  + prots = PROT_READ | (readonly ? 0 : PROT_WRITE);
>>       offset = QEMU_ALIGN_UP((uintptr_t)guardptr, align) - 
>> (uintptr_t)guardptr;
>>
>>  - ptr = mmap(guardptr + offset, size, PROT_READ | PROT_WRITE, flags, fd, 0);
>>  + ptr = mmap(guardptr + offset, size, prots, flags, fd, 0);
>>
>>       if (ptr == MAP_FAILED) {
>>           munmap(guardptr, total);
>>  diff --git a/util/oslib-posix.c b/util/oslib-posix.c
>>  index 37c5854b9c..c57466f8d9 100644
>>  --- a/util/oslib-posix.c
>>  +++ b/util/oslib-posix.c
>>  @@ -203,7 +203,7 @@ void *qemu_memalign(size_t alignment, size_t size)
>>   void *qemu_anon_ram_alloc(size_t size, uint64_t *alignment, bool shared)
>>   {
>>       size_t align = QEMU_VMALLOC_ALIGN;
>>  - void *ptr = qemu_ram_mmap(-1, size, align, shared);
>>  + void *ptr = qemu_ram_mmap(-1, size, align, shared, false);
>>
>>       if (ptr == MAP_FAILED) {
>>           return NULL;
>>  diff --git a/vl.c b/vl.c
>>  index 4c5cc0d8ad..2cc675ad21 100644
>>  --- a/vl.c
>>  +++ b/vl.c
>>  @@ -2950,6 +2950,12 @@ static void register_global_properties(MachineState 
>> *ms)
>>       user_register_global_props();
>>   }
>>
>>  +static void unprotect_ram(void *opaque)
>>  +{
>>  + error_report(__func__);
>>  + qemu_ram_unprotect_all();
>>  +}
>>  +
>>   int main(int argc, char **argv, char **envp)
>>   {
>>       int i;
>>  @@ -4537,6 +4543,8 @@ int main(int argc, char **argv, char **envp)
>>
>>       replay_start();
>>
>>  + qemu_register_reset(unprotect_ram, NULL);
>>  +
>>       /* This checkpoint is required by replay to separate prior clock
>>          reading from the other reads, because timer polling functions query
>>          clock values from the log. */
>>  --
>>  2.21.0
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK