Re: [Xen-devel] [PATCH RFC v4] x86/mm: Clean up p2m_finish_type_change return value

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 3/27/19 3:21 PM, Alexandru Stefan ISAILA wrote:
> In the case of any errors, finish_type_change() passes values returned
> from p2m->recalc() up the stack (with some exceptions in the case where
> an error is expected); this eventually ends up being returned to the
> XEN_DOMOP_map_mem_type_to_ioreq_server hypercall.
> 
> However, on Intel processors (but not on AMD processor), p2m->recalc()
> can also return '1' as well as '0'.  This case is handled very
> inconsistently: finish_type_change() will return the value of the final
> entry it attempts, discarding results for other entries;
> p2m_finish_type_change() will attempt to accumulate '1's, so that it
> returns '1' if any of the calls to finish_type_change() returns '1'; and
> dm_op() will again return '1' only if the very last call to
> p2m_finish_type_change() returns '1'.  The result is that the
> XEN_DMOP_map_mem_type_to_ioreq_server() hypercall will sometimes return
> 0 and sometimes return 1 on success, in an unpredictable manner.
> 
> The hypercall documentation doesn't mention return values; but it's not
> clear what the caller could do with the information about whether
> entries had been changed or not.  At the moment it's always 0 on AMD
> boxes, and *usually* 1 on Intel boxes; so nothing can be relying on a
> '1' return value for correctness (or if it is, it's broken).
> 
> Make the return value on success consistently '0' by only returning
> 0/-ERROR from finish_type_change().  Also remove the accumulation code
> from p2m_finish_type_change().

Thanks for putting in the effort to clean this up.

One comment: this is the second instance of the patch you posted where
this paragraph is not true.  What I wrote was meant to be an example of
how a good patch description should be written, which includes a sketch
of what the patch does technically.  You need to make sure the sketch
matches the patch.

As it happens, it looks like you'll have to modify the patch such that
v5 actually matches the description again. :-)

Also, you probably should have dropped the RFC at v2.

 -George

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