Re: kvm-27 vs 28 I/O speed

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Anthony Liguori wrote:
>>  
>>> Dave, can you diff the guest dmesg with kvm-27 and kvm-28 userspace?
>>> Maybe that will tell us something.
>>>     
>>
>> Here's kvm-27 and kvm-27+55a3212bc2...:
>>
>>
>> diff -ru kvm-fast-dmesg.txt kvm-slow-dmesg.txt
>>  Linux version 2.6.22 (dave@kernel) (gcc version 4.1.2 (Ubuntu
>> 4.1.2-0ubuntu4)) #13 Wed Jul 11 15:27:01 PDT 2007
>>  BIOS-provided physical RAM map:
>> @@ -107,20 +90,21 @@
>>  PIIX3: IDE controller at PCI slot 0000:00:01.1
>>  PIIX3: chipset revision 0
>>  PIIX3: not 100% native mode: will probe irqs later
>> -PIIX3: neither IDE port enabled (BIOS)
>> +    ide0: BM-DMA at 0x1400-0x1407, BIOS settings: hda:pio, hdb:pio
>> +    ide1: BM-DMA at 0x1408-0x140f, BIOS settings: hdc:pio, hdd:pio
>>   
>
> There's your problem.  With that patch, your kernel is using PIO
> instead of DMA.
>

Yeah, but later:

>>
>>  hda: max request size: 512KiB
>> -hda: 10485760 sectors (5368 MB) w/256KiB Cache, CHS=10402/255/63
>> +hda: 10485760 sectors (5368 MB) w/256KiB Cache, CHS=10402/255/63,
>> (U)DMA
>>  hda: cache flushes supported
>>   
>

So now the slow version uses dma.

My attempt at wild guessing is that if the port is disabled, Linux
enables it and configures it correctly.  If it is already enabled, Linux
trusts the bios to have configured it and leaves it at some silly default.

Dave, can you compare the output of hdparm -v /dev/hda?  Maybe more
clues there.


My FC6 i386 guest says:

/dev/hda:
 multcount    = 16 (on)
 IO_support   =  0 (default 16-bit)
 unmaskirq    =  0 (off)
 using_dma    =  1 (on)
 keepsettings =  0 (off)
 readonly     =  0 (off)
 readahead    = 256 (on)
 geometry     = 16383/255/63, sectors = 20971520, start = 0

Which seems nice and sane.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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