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Re: [PATCH v3 1/1] qemu-img: Add --target-is-zero to convert

From: Eric Blake
Subject: Re: [PATCH v3 1/1] qemu-img: Add --target-is-zero to convert
Date: Tue, 4 Feb 2020 08:23:59 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

On 2/4/20 7:59 AM, Vladimir Sementsov-Ogievskiy wrote:

I understand that it is safer to have restrictions now and lift them later, than to allow use of the option at any time and leave room for the user to shoot themselves in the foot with no way to add safety later.  The argument against no backing file is somewhat understandable (technically, as long as the backing file also reads as all zeroes, then the overall image reads as all zeroes - but why have a backing file that has no content?); the argument requiring -n is a bit weaker (if I'm creating an image, I _know_ it reads as all zeroes, so the --target-is-zero argument is redundant, but it shouldn't hurt to allow it).

I know that it reads as all zeroes, only if this format provides zero initialization..

+++ b/qemu-img.c

@@ -2247,6 +2256,11 @@ static int img_convert(int argc, char **argv)
          warn_report("This will become an error in future QEMU versions.");
+    if (s.has_zero_init && !skip_create) {
+        error_report("--target-is-zero requires use of -n flag");
+        goto fail_getopt;
+    }

So I think we could drop this hunk with no change in behavior.

I think, no we can't. If we allow target-is-zero, with -n, we'd better to check that what we are creating is zero-initialized (format has zero-init), and if not we should report error.

Good call. Yes, if we allow --target-is-zero without -n, we MUST insist that bdrv_has_zero_init() returns 1 (or, after my followup series, bdrv_known_zeroes() includes BDRV_ZERO_CREATE). I can work that into v2 of my series, if desired.

      s.src_num = argc - optind - 1;
      out_filename = s.src_num >= 1 ? argv[argc - 1] : NULL;
@@ -2380,6 +2394,11 @@ static int img_convert(int argc, char **argv)
      s.target_has_backing = (bool) out_baseimg;
+    if (s.has_zero_init && s.target_has_backing) {
+        error_report("Cannot use --target-is-zero with a backing file");
+        goto out;
+    }

while keeping this one makes sense.  At any rate, typo fixes are minor, and whether or not we drop the hunk I claim is a needless restriction against redundancy,

Reviewed-by: Eric Blake <address@hidden>

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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