Re: [Xen-devel] [PATCH] x86/xsave: Expose xsave_cntxt_size to avoid reloading xcr0

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.06.14 at 14:39, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 04/06/14 13:32, Jan Beulich wrote:
>>>>> On 04.06.14 at 13:56, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> xstate_ctxt_size(xfeature_mask) is runtime constant after boot.  There is no
>>> need to reload xcr0 twice for this basic bounds check.
>> Honestly I'm not in favor of exposing this variable for a not
>> performance critical code path. If we really care about saving a few
>> cycles here, how about having xstate_ctxt_size() check its argument
>> against xfeature_mask and return xsave_cntxt_size if equal?
>> Admittedly that'll need some extra care for the uses of the function
>> in xstate_init(), but it's not horribly difficult to make the current
>> function an internal one, and the global one be a wrapper with that
>> check done first.
> 
> Personally, I think that is less pleasant than exposing
> xsave_cntxt_size, but I will see about doing it.

My main concern is that the variable in no way says that this is
the size when all supported features are enabled. The function
call, otoh, nicely documents that.

Jan


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