Re: [Qemu-devel] Bug#757927: [qemu-kvm] TRIM (discard=unmap) broken in 2.1

[Michael Tokarev]

[dropping debian bugreport]

13.08.2014 10:05, Michael Tokarev wrote:
> [This is http://bugs.debian.org/757927 -- trim stopped working in qemu 2.1]
> 
> 12.08.2014 18:46, Michael Tokarev wrote:
>> 12.08.2014 17:35, Roman Mamedov wrote:
>>> Package: qemu-kvm
>>> Version: 2.1+dfsg-2~bpo70+2
>>> Severity: normal
>>>
>>> Hello,
>>>
>>> I was able to successfully use the passthrough TRIM support with an IDE
>>> interface virtual disk in Qemu-KVM version 2.0.
>>>
>>> However after upgrading to 2.1 and restarting my VM, TRIM now fails in it 
>>> with
>>> messages like those quoted below (dmesg from the guest). There are no 
>>> messages
>>> in dmesg related to that on the host.
> 
> BTW, I found a system here with an SSD which supports discard, and tried my
> guests against it -- everything works fine when I use sata/ahci, but it breaks
> indeed when using the default IDE interface.
> 
> But applying the oneliner:
> 
>> It is the same as 2.1+dfsg-1~bpo70+2 but with a one-line patch
>> added:
>>
>> diff --git a/hw/ide/core.c b/hw/ide/core.c
>> index db191a6..7256592 100644
>> --- a/hw/ide/core.c
>> +++ b/hw/ide/core.c
>> @@ -688,7 +688,8 @@ void ide_dma_cb(void *opaque, int ret)
>>             sector_num, n, s->dma_cmd);
>>  #endif
>>
>> -    if (!ide_sect_range_ok(s, sector_num, n)) {
>> +    if ((s->dma_cmd == IDE_DMA_READ || s->dma_cmd == IDE_DMA_WRITE) &&
>> +        !ide_sect_range_ok(s, sector_num, n)) {
>>          dma_buf_commit(s);
>>          ide_dma_error(s);
>>          return;
> 
> fixes the prob for me.  So apparently kwolf was right pointing to that
> commit 58ac3211.

But this makes me wonder: why the issue only exists with ide, but not
with ahci/sata?  The two should be sufficiently similar to trigger this
check the same way.  Maybe we have another bug somewhere, like adding
an extra sector in some path, which is triggered by 58ac3211?  Maybe
even in the guest kernel (but in this case, since it works with real
hw, we should ignore it).

Thanks,

/mjt