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Re: [Qemu-devel] [PATCH 03/32] vvfat: Fix partition table
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH 03/32] vvfat: Fix partition table |
Date: |
Thu, 05 Jul 2012 11:55:42 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:13.0) Gecko/20120605 Thunderbird/13.0 |
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.
>> This whole geometry calculation code in vvfat is way too convoluted to
>> understand when you haven't looked at it for some months...
>
> Yes, it is. I was briefly tempted to replace it, but then decided to
> limit my footprint in this buck-crazy block driver to minimal bug fixes.
Heh, quite understandable.
Kevin