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Re: [Xen-devel] [PATCH v2 4/6] xen/arm: cpufeature: Add helper to check constant caps



Hi,

On 09/27/2018 11:34 PM, Stefano Stabellini wrote:
On Wed, 26 Sep 2018, Julien Grall wrote:
Hi Stefano,

On 09/26/2018 05:53 PM, Stefano Stabellini wrote:
On Tue, 25 Sep 2018, Julien Grall wrote:
Some capababilities are set right during boot and will never change
afterwards. At the moment, the function cpu_have_caps will check whether
the cap is enabled from the memory.

It is possible to avoid the load from the memory by using an
ALTERNATIVE. With that the check is just reduced to 1 instruction.

Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

I enjoyed reading the patch :-)  But I don't think it is worth going
into this extreme level of optimization. test_bit is efficient enough,
right? What do you think we need to use alternatives just to check one
bit?

We already have an helper using test_bit (see cpus_have_cap). However test_bit
requires to load the word from the memory. This is an overhead when the
decision never change after boot.

One load may be insignificant by itself (thought may have a cache miss), but
if you are in hotpath this is a win in long term.

The mechanism is very similar to static key but for the poor (I don't have
much time to implement better for now). We already use similar construction on
other places (see CHECK_WORKAROUND_HELPER).

I like the mechanism and the little ALTERNATIVE trick.  I can see it
could very useful in some cases. It is just that it is a bit of a waste
to use it on SMCCC virtualization -- SMCCC calls cannot be done often
enough for this to make a difference.

I would not be so sure ;). SMC can be called at *every* entry and exit into the hypervisor to apply SSBD workaround.

While SSBD will directly call arm_smccc_1_1_smc (and not the generic one), I would not rule out more hotpath with SMC call in it.

Cheers,

--
Julien Grall

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