Re: [Xen-devel] [v5][PATCH 10/16] tools: introduce some new parameters to set rdm policy

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Tiejun Chen writes ("[v5][PATCH 10/16] tools: introduce some new parameters to 
set rdm policy"):
> This patch introduces user configurable parameters to specify RDM
> resource and according policies,
...
> Global RDM parameter, "type", allows user to specify reserved regions
> explicitly, e.g. using 'host' to include all reserved regions reported

As I understand this feature, I don't think the name `type' is right.
This is a method for handling or overriding/ignoring the RDM problem.

Perhaps `handling=' or `strategy=' ?

And `none' seems quite an innocuous name for such a dangerous
setting.  How about `ignore' ?

> 'strict/relaxed' policy decides how to handle conflict when reserving RDM
> regions in pfn space. If conflict exists, 'strict' means an immediate error
> so VM can't keep running, while 'relaxed' allows moving forward with a
> warning message thrown out.
> 
> Default per-device RDM policy is 'strict', while default global RDM policy
> is 'relaxed'. And the per-device policy would override the global policy like
> others.

I don't think these two sentences can both be true.  If the per-device
policy overrides the global policy, then the per-device policy must
have at least three values: `strict', `relaxed', and `not set'
(implicitly using global policy).

Perhaps I have misunderstood.

> diff --git a/docs/man/xl.cfg.pod.5 b/docs/man/xl.cfg.pod.5
> index a3e0e2e..a2dc343 100644
> --- a/docs/man/xl.cfg.pod.5
> +++ b/docs/man/xl.cfg.pod.5
> @@ -655,6 +655,83 @@ assigned slave device.
...
> +=item B<type="STRING">

The documentation on this is rather vague about what the default is.
But you do say:

> +In this conflict case,
> +
> +#1. If the type options is set with "none",
> +
> +rdm = "type=none,reserve=strict" or rdm = "type=none,reserve=relaxed"
> +
> +mean we don't handle any conflict just to make VM keep running as before.
> +Note this is our default behavior.

This suggests that the default is "do the dangerous thing".  That
doesn't seem right.

> +#2. If the type options is set with "host",
> +
> +rdm = "type=host,reserve=strict" or rdm = "type=host,reserve=relaxed"
...
> +"relaxed" allows a VM to be created to keep running with a warning message
> +thrown out. But this may crash this VM if this device accesses RDM. For 
> example,
> +Windows IGD GFX driver always access these regions so this lead to a blue 
> screen
> +to crash VM in such a case.

The difference between
   type=host,reserve=relaxed
and
   type=none
is simply whether a warning is printed ?

Ian.

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