Re: [Qemu-devel] [PATCH v4 2/2] memory-device: rewrite address assignment using ranges

[David Hildenbrand <david@redhat.com>]

On 13.12.18 15:48, Igor Mammedov wrote:
> On Thu, 13 Dec 2018 13:35:28 +0100
> David Hildenbrand <david@redhat.com> wrote:
> 
>> On 13.12.18 13:28, Igor Mammedov wrote:
>>> On Wed, 12 Dec 2018 10:28:21 +0100
>>> David Hildenbrand <david@redhat.com> wrote:
>>>   
>>>> Let's rewrite it properly using ranges. This fixes certain overflows that
>>>> are right now possible. E.g.
>>>>
>>>> qemu-system-x86_64 -m 4G,slots=20,maxmem=40G -M pc \
>>>>     -object memory-backend-file,id=mem1,share,mem-path=/dev/zero,size=2G
>>>>     -device pc-dimm,memdev=mem1,id=dimm1,addr=-0x40000000
>>>>
>>>> Now properly errors out instead of succeeding. (Note that qapi
>>>> parsing of huge uint64_t values is broken and fixes are on the way)
>>>>
>>>> "can't add memory device [0xffffffffa0000000:0x80000000], usable range for
>>>> memory devices [0x140000000:0xe00000000]"
>>>>
>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>> ---
>>>>  hw/mem/memory-device.c | 54 ++++++++++++++++++++++++------------------
>>>>  1 file changed, 31 insertions(+), 23 deletions(-)
>>>>
>>>> diff --git a/hw/mem/memory-device.c b/hw/mem/memory-device.c
>>>> index 8be63c8032..28e871f562 100644
>>>> --- a/hw/mem/memory-device.c
>>>> +++ b/hw/mem/memory-device.c
>>>> @@ -100,9 +100,8 @@ static uint64_t memory_device_get_free_addr(MachineState *ms,
>>>>                                              uint64_t align, uint64_t size,
>>>>                                              Error **errp)
>>>>  {
>>>> -    uint64_t address_space_start, address_space_end;
>>>>      GSList *list = NULL, *item;
>>>> -    uint64_t new_addr = 0;
>>>> +    Range as, new = range_empty;
>>>>  
>>>>      if (!ms->device_memory) {
>>>>          error_setg(errp, "memory devices (e.g. for memory hotplug) are not "
>>>> @@ -115,13 +114,11 @@ static uint64_t memory_device_get_free_addr(MachineState *ms,
>>>>                           "enabled, please specify the maxmem option");
>>>>          return 0;
>>>>      }
>>>> -    address_space_start = ms->device_memory->base;
>>>> -    address_space_end = address_space_start +
>>>> -                        memory_region_size(&ms->device_memory->mr);
>>>> -    g_assert(address_space_end >= address_space_start);
>>>> +    range_init_nofail(&as, ms->device_memory->base,
>>>> +                      memory_region_size(&ms->device_memory->mr));
>>>>  
>>>> -    /* address_space_start indicates the maximum alignment we expect */
>>>> -    if (!QEMU_IS_ALIGNED(address_space_start, align)) {
>>>> +    /* start of address space indicates the maximum alignment we expect */
>>>> +    if (!QEMU_IS_ALIGNED(range_lob(&as), align)) {
>>>>          error_setg(errp, "the alignment (0x%" PRIx64 ") is not supported",
>>>>                     align);
>>>>          return 0;
>>>> @@ -145,20 +142,25 @@ static uint64_t memory_device_get_free_addr(MachineState *ms,
>>>>      }
>>>>  
>>>>      if (hint) {
>>>> -        new_addr = *hint;
>>>> -        if (new_addr < address_space_start) {
>>>> +        if (range_init(&new, *hint, size)) {
>>>>              error_setg(errp, "can't add memory device [0x%" PRIx64 ":0x%" PRIx64
>>>> -                       "] before 0x%" PRIx64, new_addr, size,
>>>> -                       address_space_start);
>>>> +                       "], usable range for memory devices [0x%" PRIx64 ":0x%"
>>>> +                       PRIx64 "]", *hint, size, range_lob(&as),
>>>> +                       range_size(&as));  
>>> this changes error message to be the same as the next one and looses 'before' meaning
>>> so if you'd like to have the same error message, then prbably merging both branches would be better.  
>>
>> I can do that, but I'll have to refer to "*hint" and "size" then instead
>> of range_lob(&new) and range_size(&new), because the range might not be
>> initialized.
> either that or better make errors different to avoid confusion.
> 

Will see what turns out better. As we indicate the ranges the user can
figure out what is going wrong.

> [...]
>>>> -            new_addr = QEMU_ALIGN_UP(md_addr + md_size, align);
>>>> +
>>>> +            next_addr = QEMU_ALIGN_UP(range_upb(&tmp) + 1, align);
>>>> +            if (!next_addr || range_init(&new, next_addr, range_size(&new))) {
>>>> +                range_make_empty(&new);
>>>> +                break;
>>>> +            }
>>>>          }
>>>>      }
>>>>  
>>>> -    if (new_addr + size > address_space_end) {
>>>> +    if (!range_contains_range(&as, &new)) {
>>>>          error_setg(errp, "could not find position in guest address space for "
>>>>                     "memory device - memory fragmented due to alignments");  
>>> it could happen due to fragmentation but also in case remaining free space is no enough  
>>
>> That should be handled via memory_device_check_addable(), which is
>> called at the beginning of the function. It checks for general size
>> availability.
> 
> I've meant
>  AS_LOB                 AS_UPB
>    100                   1000
>  MEM1_LOB  MEM1_UPB
>    100     900
> then hotplugging MEM2 with size 200 would fail with this message,
> which could be a bit confusing.
> Maybe "not enough space to plug device of size %d" would be better?


That should be covered by memory_device_check_addable() if I am not wrong.

used_region_size + size > ms->maxram_size - ms->ram_size

For your example (if I don't mess up the numbers):

ms->maxram_size - ms->ram_size = 900
used_region_size = 800

So trying to add anything > 100 will bail out.


>    
>>
>> Thanks!
>>
> 


-- 

Thanks,

David / dhildenb