Re: [Qemu-devel] [PATCH] pflash: Only read non-zero parts of backend image

[Laszlo Ersek <lersek@redhat.com>]

Hi Markus,

On 05/07/19 20:01, Markus Armbruster wrote:
> The subject is slightly misleading.  Holes read as zero.  So do
> non-holes full of zeroes.  The patch avoids reading the former, but
> still reads the latter.
> 
> Xiang Zheng <zhengxiang9@huawei.com> writes:
> 
>> Currently we fill the memory space with two 64MB NOR images when
>> using persistent UEFI variables on virt board. Actually we only use
>> a very small(non-zero) part of the memory while the rest significant
>> large(zero) part of memory is wasted.
> 
> Neglects to mention that the "virt board" is ARM.
> 
>> So this patch checks the block status and only writes the non-zero part
>> into memory. This requires pflash devices to use sparse files for
>> backends.
> 
> I started to draft an improved commit message, but then I realized this
> patch can't work.
> 
> The pflash_cfi01 device allocates its device memory like this:
> 
>     memory_region_init_rom_device(
>         &pfl->mem, OBJECT(dev),
>         &pflash_cfi01_ops,
>         pfl,
>         pfl->name, total_len, &local_err);
> 
> pflash_cfi02 is similar.
> 
> memory_region_init_rom_device() calls
> memory_region_init_rom_device_nomigrate() calls qemu_ram_alloc() calls
> qemu_ram_alloc_internal() calls g_malloc0().  Thus, all the device
> memory gets written to even with this patch.

As far as I can see, qemu_ram_alloc_internal() calls g_malloc0() only to
allocate the the new RAMBlock object called "new_block". The actual
guest RAM allocation occurs inside ram_block_add(), which is also called
by qemu_ram_alloc_internal().

One frame outwards the stack, qemu_ram_alloc() passes NULL to
qemu_ram_alloc_internal(), for the 4th ("host") parameter. Therefore, in
qemu_ram_alloc_internal(), we set "new_block->host" to NULL as well.

Then in ram_block_add(), we take the (!new_block->host) branch, and call
phys_mem_alloc().

Unfortunately, "phys_mem_alloc" is a function pointer, set with
phys_mem_set_alloc(). The phys_mem_set_alloc() function is called from
"target/s390x/kvm.c" (setting the function pointer to
legacy_s390_alloc()), so it doesn't apply in this case. Therefore we end
up calling the default qemu_anon_ram_alloc() function, through the
funcptr. (I think anyway.)

And qemu_anon_ram_alloc() boils down to mmap() + MAP_ANONYMOUS, in
qemu_ram_mmap(). (Even on PPC64 hosts, because qemu_anon_ram_alloc()
passes (-1) for "fd".)

I may have missed something, of course -- I obviously didn't test it,
just speculated from the source.

Thanks
Laszlo

> 
> I'm afraid you neglected to test.
> 
> I still believe this approach can be made to work.  Need a replacement
> for memory_region_init_rom_device() that uses mmap() with MAP_ANONYMOUS.
>