Am 05.02.2013 12:10, schrieb Stefan Weil: > Am 05.02.2013 11:54, schrieb Kevin Wolf: >> Am 01.02.2013 22:51, schrieb Stefan Weil: >>> The size calculated from the CHS values is not the real image (disk) size, >>> but usually a smaller value (this is caused by rounding effects). >>> >>> Only older operating systems use CHS. Such guests won't be able to use >>> the whole disk. >>> >>> This patch fixes https://bugs.launchpad.net/qemu/+bug/1105670/. >>> >>> Signed-off-by: Stefan Weil <s...@weilnetz.de> >>> --- >>> block/vpc.c | 9 ++++----- >>> 1 file changed, 4 insertions(+), 5 deletions(-) >>> >>> diff --git a/block/vpc.c b/block/vpc.c >>> index fff103b..799b1c9 100644 >>> --- a/block/vpc.c >>> +++ b/block/vpc.c >>> @@ -198,11 +198,10 @@ static int vpc_open(BlockDriverState *bs, int flags) >>> /* Write 'checksum' back to footer, or else will leave it with zero. */ >>> footer->checksum = be32_to_cpu(checksum); >>> >>> - // The visible size of a image in Virtual PC depends on the geometry >>> - // rather than on the size stored in the footer (the size in the footer >>> - // is too large usually) >>> - bs->total_sectors = (int64_t) >>> - be16_to_cpu(footer->cyls) * footer->heads * footer->secs_per_cyl; >>> + /* The visible size of an image in Virtual PC may depend on the >>> geometry >>> + * rather than on the size stored in the footer (the size in the footer >>> + * is usually larger). Nevertheless we must use the real size here. */ >>> + bs->total_sectors = be64_to_cpu(footer->size) / 512; >>> >>> /* Allow a maximum disk size of approximately 2 TB */ >>> if (bs->total_sectors >= 65535LL * 255 * 255) { >> Now VMs created with qemu can't be run in Virtual PC any more because >> they use more space than is really visible on VPC. > > Now VMs created with QEMU show the size which is also > reported by MS tools. I don't think this is a problem. > > >> The bug report doesn't say anything about practical consequences of the >> unexpected behaviour, so I'd tend to claim that just the expectation was >> wrong. Having to deal with two differing sizes is ugly, but we had the >> discussion before and using the geometry was considered the least bad >> way, because it gives you the same disk size within the VM as on Virtual PC. >> >> If you want to change it, you need to provide a real use case that breaks. >> >> Kevin > > Test case: > > * Create a VHD disk image (either with QEMU or MS tools). > * Run a modern guest OS (which does not use CHS) on this image > using Virtual PC or VirtualBox and write into the last disk blocks. > * Copy the image to another format using qemu-img. > > => The last blocks are missing now.
When I wrote that code it wasn't like this. I doubt that Linux was relying on the geometry until recently, so is this a change in the behaviour of newer Virtual PC versions? If yes, that would be very nasty to deal with. I'm very sure that back then I needed to use the geometry so that images could be exchanged between qemu and VPC. If I didn't, they would either get truncated or appear to be larger on one of both. Kevin
