Re: [Xen-devel] [PATCH v2 07/17] libxc/restore: STATIC_DATA_END inference for v2 compatibility

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 24/02/2020 17:32, Ian Jackson wrote:
> Andrew Cooper writes ("[PATCH v2 07/17] libxc/restore: STATIC_DATA_END inference for v2 compatibility"):
>> A v3 stream can compatibly read a v2 stream by inferring the position of the
>> STATIC_DATA_END record.
>>
>> v2 compatibility is only needed for x86.  No other architectures exist yet,
>> but they will have a minimum of v3 when introduced.
>>
>> The x86 HVM compatibility point being in handle_page_data() (which is common
>> code) is a bit awkward.  However, as the two compatibility points are subtly
>> different, and it is (intentionally) not possible to call into arch specific
>> code from common code (except via the ops hooks), use some #ifdef-ary and
>> opencode the check, rather than make handle_page_data() a per-arch helper.
> ...
>> +#if defined(__i386__) || defined(__x86_64__)
>> +    /* v2 compat.  Infer the position of STATIC_DATA_END. */
>> +    if ( ctx->restore.format_version < 3 && !ctx->restore.seen_static_data_end )
>> +    {
>> +        rc = handle_static_data_end(ctx);
>> +        if ( rc )
> These 17 lines appears twice, in basically identical form.  Could it
> be refactored ?

Not really, no.

The error handling (i.e. half of those 17 lines) is different, making it
somewhat awkward to fit into a static inline.

More importantly however, by design, common code can't call
arch-specific code without a restore_ops hook.  Deduping these would
require breaking the restriction which is currently doing a decent job
of keeping x86-isms out of common code.

~Andrew

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