Re: [Qemu-devel] [PATCH for-2.5 7/8] s390x: Migrate guest storage keys (initial memory only)

[David Hildenbrand <dahi@linux.vnet.ibm.com>]

> 
> So if I've got this code right, you send here a "header" that announces
> a packet with all pages ...
> 
> > +    while (handled_count < total_count) {
> > +        cur_count = MIN(total_count - handled_count, S390_SKEYS_BUFFER_SIZE);
> > +
> > +        ret = skeyclass->get_skeys(ss, cur_gfn, cur_count, buf);
> > +        if (ret < 0) {
> > +            error_report("S390_GET_KEYS error %d\n", ret);
> > +            break;
> 
> ... but when an error occurs here, you suddenly stop in the middle of
> that "packet" with all pages ...

Indeed, although that should never fail, we never know.
We don't want to overengineer the protocol but still abort migration at least
on the loading side in that (theoretical) case.

> 
> > +        }
> > +
> > +        /* write keys to stream */
> > +        qemu_put_buffer(f, buf, cur_count);
> > +
> > +        cur_gfn += cur_count;
> > +        handled_count += cur_count;
> > +    }
> > +
> > +    g_free(buf);
> > +end_stream:
> > +    qemu_put_be64(f, S390_SKEYS_SAVE_FLAG_EOS);
> 
> ... and send an EOS marker here instead ...
> 
> > +}
> > +
> > +static int s390_storage_keys_load(QEMUFile *f, void *opaque, int version_id)
> > +{
> > +    S390SKeysState *ss = S390_SKEYS(opaque);
> > +    S390SKeysClass *skeyclass = S390_SKEYS_GET_CLASS(ss);
> > +    int ret = 0;
> > +
> > +    while (!ret) {
> > +        ram_addr_t addr;
> > +        int flags;
> > +
> > +        addr = qemu_get_be64(f);
> > +        flags = addr & ~TARGET_PAGE_MASK;
> > +        addr &= TARGET_PAGE_MASK;
> > +
> > +        switch (flags) {
> > +        case S390_SKEYS_SAVE_FLAG_SKEYS: {
> > +            const uint64_t total_count = qemu_get_be64(f);
> > +            uint64_t handled_count = 0, cur_count;
> > +            uint64_t cur_gfn = addr / TARGET_PAGE_SIZE;
> > +            uint8_t *buf = g_try_malloc(S390_SKEYS_BUFFER_SIZE);
> > +
> > +            if (!buf) {
> > +                error_report("storage key load could not allocate memory\n");
> > +                ret = -ENOMEM;
> > +                break;
> > +            }
> > +
> > +            while (handled_count < total_count) {
> > +                cur_count = MIN(total_count - handled_count,
> > +                                S390_SKEYS_BUFFER_SIZE);
> > +                qemu_get_buffer(f, buf, cur_count);
> 
> ... while the receiver can not handle the EOS marker here.
> 
> This looks fishy to me (or I might have just missed something), but
> anyway please double check whether your error handling in the sender
> really makes sense.

My shot would be, to send invalid storage keys if getting the keys from the
kernel fails. So we can detect it on the loading side and abort migration
gracefully.

> 
> > +                ret = skeyclass->set_skeys(ss, cur_gfn, cur_count, buf);
> > +                if (ret < 0) {
> > +                    error_report("S390_SET_KEYS error %d\n", ret);
> > +                    break;
> > +                }
> > +                handled_count += cur_count;
> > +                cur_gfn += cur_count;
> > +            }
> > +            g_free(buf);
> > +            break;
> > +        }
> > +        case S390_SKEYS_SAVE_FLAG_EOS:
> > +            /* normal exit */
> > +            return 0;
> > +        default:
> > +            error_report("Unexpected storage key flag data: %#x", flags);
> > +            ret = -EINVAL;
> > +        }
> > +    }
> > +
> > +    return ret;
> > +}
> 
>  Thomas

Thanks Thomas!


David