Re: [Xen-devel] [PATCH V5 31/32] libxl: update domain configuration when updating memory targets

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Wei Liu writes ("[PATCH V5 31/32] libxl: update domain configuration when 
updating memory targets"):
> Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
...
> @@ -4005,6 +4024,14 @@ int libxl_domain_setmaxmem(libxl_ctx *ctx, uint32_t 
> domid, uint32_t max_memkb)
>          goto out;
>      }
>  
> +    {
> +        LOAD_DOMAIN_CONFIG(domid);
> +
> +        d_config.b_info.max_memkb = max_memkb;
> +
> +        STORE_DOMAIN_CONFIG(domid);

I think this is a fundamentally hazardous way to deal with domain
configuration updates.

The result is that there are two places where the domain's maximum
memory is recorded - the actual running domain state, and the saved
json configuration.  In error situations it is possible for these to
become out of step.

What you should do instead is have the domain configuration retrieval
code (currently known as libxl_load_domain_configuration) do
xc_domain_getmaxmem (or whatever it is) and update the value in the
config data it is about to return.

That way there is only one place where the maxmem information is
stored.  (There is of course another copy in the json config state but
it will always be ignored so can be disregarded.)  So it is not
possible for the system to be in an inconsistent state.

The same considerations apply to device addition and removal, vif MAC
address update, etc.

Ian.

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