Re: [PATCH v2 8/8] x86/PV: use get_unsafe() instead of copy_from_unsafe()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.02.2021 16:37, Roger Pau Monné wrote:
> On Tue, Feb 23, 2021 at 04:25:00PM +0100, Jan Beulich wrote:
>> On 23.02.2021 12:59, Roger Pau Monné wrote:
>>> On Wed, Feb 17, 2021 at 09:23:33AM +0100, Jan Beulich wrote:
>>>> The former expands to a single (memory accessing) insn, which the latter
>>>> does not guarantee. Yet we'd prefer to read consistent PTEs rather than
>>>> risking a split read racing with an update done elsewhere.
>>>>
>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>
>>> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>>
>>> Albeit I wonder why the __builtin_constant_p check done in
>>> copy_from_unsafe is not enough to take the get_unsafe_size branch in
>>> there. Doesn't sizeof(l{1,2}_pgentry_t) qualify as a built time
>>> constant?
>>>
>>> Or the fact that n it's a parameter to an inline function hides this,
>>> in which case the __builtin_constant_p is pointless?
>>
>> Without (enough) optimization, __builtin_constant_p() may indeed
>> yield false in such cases. But that wasn't actually what I had
>> in mind when making this change (and the original similar on in
>> shadow code). Instead, at the time I made the shadow side change,
>> I had removed this optimization from the new function flavors.
>> With that removal, things are supposed to still be correct - it's
>> an optimization only, after all. Meanwhile the optimizations are
>> back, so there's no immediate problem as long as the optimizer
>> doesn't decide to out-of-line the function invocations (we
>> shouldn't forget that even always_inline is not a guarantee for
>> inlining to actually occur).
> 
> I'm fine with you switching those use cases to get_unsafe, but I think
> the commit message should be slightly adjusted to notice that
> copy_from_unsafe will likely do the right thing, but that it's simply
> clearer to call get_unsafe directly, also in case copy_from_unsafe
> gets changed in the future to drop the get_unsafe paths.

How about this then?

"The former expands to a single (memory accessing) insn, which the latter
 does not guarantee (the __builtin_constant_p() based switch() statement
 there is just an optimization). Yet we'd prefer to read consistent PTEs
 rather than risking a split read racing with an update done elsewhere."

Jan