Re: [PATCH 0/6] xsm: refactoring xsm hooks

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 18/06/2021 12:48, Jan Beulich wrote:
> On 18.06.2021 12:14, Andrew Cooper wrote:
>> On 18/06/2021 00:39, Daniel P. Smith wrote:
>>> Based on feedback from 2021 Xen Developers Summit the xsm-roles RFC
>>> patch set is being split into two separate patch sets. This is the first
>>> patch set and is focused purely on the clean up and refactoring of the
>>> XSM hooks.
>>>
>>> This patch set refactors the xsm_ops wrapper hooks to use the alternative_call
>>> infrastructure. Then proceeds to move and realign the headers to remove the
>>> psuedo is/is not enable implementation. The remainder of the changes are clean up
>>> and removing no longer necessary abstractions.
>>>
>>> <snip>
>>>  51 files changed, 1309 insertions(+), 1413 deletions(-)
>> The diffstat is great, but sadly CI says no. 
>> https://gitlab.com/xen-project/patchew/xen/-/pipelines/323044913
>>
>> The problem is that ARM doesn't have alternative_vcall().  Given how
>> much of an improvement this ought to be for hypercalls, I don't want to
>> lose the vcalls.
>>
>> One option is to implement vcall() support on ARM, but that will leave
>> new architectures (RISC-V on the way) with a heavy lift to get XSM to
>> compile.
>>
>> Instead, what we want to do is make vcall() a common interface, falling
>> back to a plain function pointer call for architectures which don't
>> implement the optimisation.  So something like:
>>
>> 1) Introduce CONFIG_HAS_VCALL, which is selected by X86 only right now
>> 2) Introduce xen/vcall.h which uses CONFIG_HAS_VCALL to either include
>> asm/vcall.h or provide the fallback implementation
> A word on the suggested names: The 'v' in alternative_vcall() stands for
> "returning void", as opposed to alternative_call(). It's unclear to me
> what you see it stand for in the names you propose.

Urgh - yet another reason to prefer the Linux static_call() infrastructure.

Would a general alt_call name be acceptable?

~Andrew