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Re: [Qemu-devel] [PATCH v3 03/22] qcow2: Use 64 bits for refcount values
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH v3 03/22] qcow2: Use 64 bits for refcount values |
Date: |
Thu, 27 Nov 2014 13:49:53 +0000 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
On Thu, Nov 20, 2014 at 06:06:19PM +0100, Max Reitz wrote:
> Refcounts may have a width of up to 64 bits, so qemu should use the same
> width to represent refcount values internally.
>
> Since for instance qcow2_get_refcount() signals an error by returning a
> negative value, refcount values are generally signed to be able to
> represent those error values correctly. This limits the maximum refcount
> value supported by qemu to INT64_MAX (= 63 bits), as established in
> "qcow2: Add two new fields to BDRVQcowState".
>
> This limitation should have no implications in practice for normal valid
> images. If the MSb in a 64 bit refcount value is set, we can safely
> assume the value to be invalid (because reaching such high refcounts is
> impossible due to other limitations of the qcow2 format).
>
> Signed-off-by: Max Reitz <address@hidden>
> Reviewed-by: Eric Blake <address@hidden>
> ---
> block/qcow2-cluster.c | 9 ++++++---
> block/qcow2-refcount.c | 37 ++++++++++++++++++++-----------------
> block/qcow2.h | 7 ++++---
> 3 files changed, 30 insertions(+), 23 deletions(-)
Reviewed-by: Stefan Hajnoczi <address@hidden>
pgp07TDJp5C2y.pgp
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- [Qemu-devel] [PATCH v3 00/22] qcow2: Support refcount orders != 4, Max Reitz, 2014/11/20
- [Qemu-devel] [PATCH v3 01/22] qcow2: Add two new fields to BDRVQcowState, Max Reitz, 2014/11/20
- [Qemu-devel] [PATCH v3 03/22] qcow2: Use 64 bits for refcount values, Max Reitz, 2014/11/20
- Re: [Qemu-devel] [PATCH v3 03/22] qcow2: Use 64 bits for refcount values,
Stefan Hajnoczi <=
- [Qemu-devel] [PATCH v3 04/22] qcow2: Respect error in qcow2_alloc_bytes(), Max Reitz, 2014/11/20
- [Qemu-devel] [PATCH v3 05/22] qcow2: Refcount overflow and qcow2_alloc_bytes(), Max Reitz, 2014/11/20
- [Qemu-devel] [PATCH v3 06/22] qcow2: Helper for refcount array reallocation, Max Reitz, 2014/11/20