Re: [PATCH RESEND v6 01/36] memory: alloc RAM from file at offset

[Jag Raman <jag.raman@oracle.com>]

> On May 12, 2020, at 4:48 AM, Daniel P. Berrangé <berrange@redhat.com> wrote:
> 
> On Wed, Apr 22, 2020 at 09:13:36PM -0700, elena.ufimtseva@oracle.com wrote:
>> From: Jagannathan Raman <jag.raman@oracle.com>
>> 
>> Allow RAM MemoryRegion to be created from an offset in a file, instead
>> of allocating at offset of 0 by default. This is needed to synchronize
>> RAM between QEMU & remote process.
> 
> Can you elaborate on why remote processes require the RAM to be offset
> from zero ?

Hi Daniel,

As it turns out, the RAM is scattered across the physical address space
(system_memory) of QEMU. Therefore, the system memory is composed
of multiple sections of RAM, and some sections start at a non-zero RAM
offset.

As a result, the remote process needs the ability to map these RAM
sections into system_memory.

Thank you!
--
Jag

> 
> NB, I'm not objecting - I'm just curious to understand more.
> 
>> 
>> Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
>> Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
>> Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
>> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
>> ---
>> exec.c                    | 11 +++++++----
>> include/exec/ram_addr.h   |  2 +-
>> include/qemu/mmap-alloc.h |  3 ++-
>> memory.c                  |  2 +-
>> util/mmap-alloc.c         |  7 ++++---
>> util/oslib-posix.c        |  2 +-
>> 6 files changed, 16 insertions(+), 11 deletions(-)
>> 
>> diff --git a/exec.c b/exec.c
>> index 2874bb5088..d0ac9545f4 100644
>> --- a/exec.c
>> +++ b/exec.c
>> @@ -1801,6 +1801,7 @@ static void *file_ram_alloc(RAMBlock *block,
>>                             ram_addr_t memory,
>>                             int fd,
>>                             bool truncate,
>> +                            off_t offset,
>>                             Error **errp)
>> {
>>     void *area;
>> @@ -1851,7 +1852,8 @@ static void *file_ram_alloc(RAMBlock *block,
>>     }
>> 
>>     area = qemu_ram_mmap(fd, memory, block->mr->align,
>> -                         block->flags & RAM_SHARED, block->flags & RAM_PMEM);
>> +                         block->flags & RAM_SHARED, block->flags & RAM_PMEM,
>> +                         offset);
>>     if (area == MAP_FAILED) {
>>         error_setg_errno(errp, errno,
>>                          "unable to map backing store for guest RAM");
>> @@ -2283,7 +2285,7 @@ static void ram_block_add(RAMBlock *new_block, Error **errp, bool shared)
>> #ifdef CONFIG_POSIX
>> RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
>>                                  uint32_t ram_flags, int fd,
>> -                                 Error **errp)
>> +                                 off_t offset, Error **errp)
>> {
>>     RAMBlock *new_block;
>>     Error *local_err = NULL;
>> @@ -2328,7 +2330,8 @@ RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
>>     new_block->used_length = size;
>>     new_block->max_length = size;
>>     new_block->flags = ram_flags;
>> -    new_block->host = file_ram_alloc(new_block, size, fd, !file_size, errp);
>> +    new_block->host = file_ram_alloc(new_block, size, fd, !file_size, offset,
>> +                                     errp);
>>     if (!new_block->host) {
>>         g_free(new_block);
>>         return NULL;
>> @@ -2358,7 +2361,7 @@ RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
>>         return NULL;
>>     }
>> 
>> -    block = qemu_ram_alloc_from_fd(size, mr, ram_flags, fd, errp);
>> +    block = qemu_ram_alloc_from_fd(size, mr, ram_flags, fd, 0, errp);
>>     if (!block) {
>>         if (created) {
>>             unlink(mem_path);
>> diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
>> index 5e59a3d8d7..1b9f489ff0 100644
>> --- a/include/exec/ram_addr.h
>> +++ b/include/exec/ram_addr.h
>> @@ -121,7 +121,7 @@ RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
>>                                    Error **errp);
>> RAMBlock *qemu_ram_alloc_from_fd(ram_addr_t size, MemoryRegion *mr,
>>                                  uint32_t ram_flags, int fd,
>> -                                 Error **errp);
>> +                                 off_t offset, Error **errp);
>> 
>> RAMBlock *qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
>>                                   MemoryRegion *mr, Error **errp);
>> diff --git a/include/qemu/mmap-alloc.h b/include/qemu/mmap-alloc.h
>> index e786266b92..4f579858bc 100644
>> --- a/include/qemu/mmap-alloc.h
>> +++ b/include/qemu/mmap-alloc.h
>> @@ -25,7 +25,8 @@ void *qemu_ram_mmap(int fd,
>>                     size_t size,
>>                     size_t align,
>>                     bool shared,
>> -                    bool is_pmem);
>> +                    bool is_pmem,
>> +                    off_t start);
>> 
>> void qemu_ram_munmap(int fd, void *ptr, size_t size);
>> 
>> diff --git a/memory.c b/memory.c
>> index 601b749906..f5fec476b7 100644
>> --- a/memory.c
>> +++ b/memory.c
>> @@ -1596,7 +1596,7 @@ void memory_region_init_ram_from_fd(MemoryRegion *mr,
>>     mr->destructor = memory_region_destructor_ram;
>>     mr->ram_block = qemu_ram_alloc_from_fd(size, mr,
>>                                            share ? RAM_SHARED : 0,
>> -                                           fd, &err);
>> +                                           fd, 0, &err);
>>     mr->dirty_log_mask = tcg_enabled() ? (1 << DIRTY_MEMORY_CODE) : 0;
>>     if (err) {
>>         mr->size = int128_zero();
>> diff --git a/util/mmap-alloc.c b/util/mmap-alloc.c
>> index 27dcccd8ec..a28f7025f0 100644
>> --- a/util/mmap-alloc.c
>> +++ b/util/mmap-alloc.c
>> @@ -86,7 +86,8 @@ void *qemu_ram_mmap(int fd,
>>                     size_t size,
>>                     size_t align,
>>                     bool shared,
>> -                    bool is_pmem)
>> +                    bool is_pmem,
>> +                    off_t start)
>> {
>>     int flags;
>>     int map_sync_flags = 0;
>> @@ -147,7 +148,7 @@ void *qemu_ram_mmap(int fd,
>>     offset = QEMU_ALIGN_UP((uintptr_t)guardptr, align) - (uintptr_t)guardptr;
>> 
>>     ptr = mmap(guardptr + offset, size, PROT_READ | PROT_WRITE,
>> -               flags | map_sync_flags, fd, 0);
>> +               flags | map_sync_flags, fd, start);
>> 
>>     if (ptr == MAP_FAILED && map_sync_flags) {
>>         if (errno == ENOTSUP) {
>> @@ -172,7 +173,7 @@ void *qemu_ram_mmap(int fd,
>>          * we will remove these flags to handle compatibility.
>>          */
>>         ptr = mmap(guardptr + offset, size, PROT_READ | PROT_WRITE,
>> -                   flags, fd, 0);
>> +                   flags, fd, start);
>>     }
>> 
>>     if (ptr == MAP_FAILED) {
>> diff --git a/util/oslib-posix.c b/util/oslib-posix.c
>> index 062236a1ab..4c6b9e90c6 100644
>> --- a/util/oslib-posix.c
>> +++ b/util/oslib-posix.c
>> @@ -209,7 +209,7 @@ 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, false);
>> +    void *ptr = qemu_ram_mmap(-1, size, align, shared, false, 0);
>> 
>>     if (ptr == MAP_FAILED) {
>>         return NULL;
>> -- 
>> 2.25.GIT
>> 
> 
> Regards,
> Daniel
> -- 
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> 
>