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Re: [Qemu-devel] [PATCH 03/32] vvfat: Fix partition table
From: |
Markus Armbruster |
Subject: |
Re: [Qemu-devel] [PATCH 03/32] vvfat: Fix partition table |
Date: |
Thu, 05 Jul 2012 13:10:07 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/23.3 (gnu/linux) |
Kevin Wolf <address@hidden> writes:
> Am 05.07.2012 11:23, schrieb Markus Armbruster:
>> Kevin Wolf <address@hidden> writes:
>>
>>> Am 29.06.2012 22:33, schrieb Blue Swirl:
>>>> On Fri, Jun 29, 2012 at 3:34 PM, Markus Armbruster <address@hidden> wrote:
>>>>> Unless parameter ":floppy:" is given, vvfat creates a virtual image
>>>>> with DOS MBR defining a single partition which holds the FAT file
>>>>> system. The size of the virtual image depends on the width of the
>>>>> FAT: 32 MiB (CHS 64, 16, 63) for 12 bit FAT, 504 MiB (CHS 1024, 16,
>>>>> 63) for 16 and 32 bit FAT, leaving (64*16-1)*63 = 64449 and
>>>>> (1024*16-1)*64 = 1032129 sectors for the partition.
>>>>>
>>>>> However, it screws up the end of the partition in the MBR:
>>>>>
>>>>> FAT width param. start CHS end CHS start LBA size
>>>>> :32: 0,1,1 1023,14,63 63 1032065
>>>>> :16: 0,1,1 1023,14,55 63 1032057
>>>>> :12: 0,1,1 63,14,55 63 64377
>>>>>
>>>>> The actual FAT file system nevertheless assumes the partition has
>>>>> 1032129 or 64449 sectors. Oops.
>>>>>
>>>>> Signed-off-by: Markus Armbruster <address@hidden>
>>>>> ---
>>>>> block/vvfat.c | 7 ++++---
>>>>> 1 files changed, 4 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/block/vvfat.c b/block/vvfat.c
>>>>> index 0fd3367..62745b5 100644
>>>>> --- a/block/vvfat.c
>>>>> +++ b/block/vvfat.c
>>>>> @@ -394,11 +394,12 @@ static void init_mbr(BDRVVVFATState* s)
>>>>>
>>>>> /* LBA is used when partition is outside the CHS geometry */
>>>>> lba = sector2CHS(s->bs, &partition->start_CHS,
>>>>> s->first_sectors_number-1);
>>>>> - lba|= sector2CHS(s->bs, &partition->end_CHS, s->sector_count);
>>>>> + lba |= sector2CHS(s->bs, &partition->end_CHS, s->bs->total_sectors -
>>>>> 1);
>>>>>
>>>>> /*LBA partitions are identified only by start/length_sector_long not
>>>>> by CHS*/
>>>>> - partition->start_sector_long =cpu_to_le32(s->first_sectors_number-1);
>>>>> - partition->length_sector_long=cpu_to_le32(s->sector_count -
>>>>> s->first_sectors_number+1);
>>>>> + partition->start_sector_long =
>>>>> cpu_to_le32(s->first_sectors_number-1);
>>>>
>>>> Spaces around '-'. Thanks for fixing the other cases, BTW.
>>>
>>> For compensation there's an extra space before the '='.
>>
>> The original lined up the two '='. I preserved that. Not that I care
>> for it. Want me to drop the extra space?
>
> Ah, didn't notice that. I don't mind then.
>
>>>>> + partition->length_sector_long = cpu_to_le32(s->bs->total_sectors
>>>>> + -
>>>>> s->first_sectors_number + 1);
>>>
>>> Just wondering... This should be the same as s->sector_count, right?
>>
>> Hmm. vvfat_open() assigns:
>>
>> s->sector_count = cyls * heads * secs - (s->first_sectors_number - 1);
>> bs->total_sectors = cyls * heads * secs;
>>
>> But it then changes it minds and does:
>>
>> s->sector_count = s->faked_sectors +
>> s->sectors_per_cluster*s->cluster_count;
>
> Which probably means that they differ if some sub-cluster sized space is
> left unused at the end of the disk. It's not useful to have this space
> included in the partition, but it doesn't hurt either. So I suppose
> either way is fine.
Complication: the partition should end on a cylinder boundary. Shaving
off an unused tail may well interfere with that. Let's stick to v1
here.
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