Re: [PATCH v2 00/15] xen: drop hypercall function tables

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.11.2021 15:18, Andrew Cooper wrote:
> On 01/11/2021 15:20, Juergen Gross wrote:
>> In order to avoid indirect function calls on the hypercall path as
>> much as possible this series is removing the hypercall function tables
>> and is replacing the hypercall handler calls via the function array
>> by automatically generated call macros.
>>
>> Another by-product of generating the call macros is the automatic
>> generating of the hypercall handler prototypes from the same data base
>> which is used to generate the macros.
>>
>> This has the additional advantage of using type safe calls of the
>> handlers and to ensure related handler (e.g. PV and HVM ones) share
>> the same prototypes.
>>
>> A very brief performance test (parallel build of the Xen hypervisor
>> in a 6 vcpu guest) showed a very slim improvement (less than 1%) of
>> the performance with the patches applied. The test was performed using
>> a PV and a PVH guest.
>>
>> Changes in V2:
>> - new patches 6, 14, 15
>> - patch 7: support hypercall priorities for faster code
>> - comments addressed
>>
>> Juergen Gross (15):
>>   xen: limit number of hypercall parameters to 5
>>   xen: move do_vcpu_op() to arch specific code
>>   xen: harmonize return types of hypercall handlers
>>   xen/x86: modify hvm_memory_op() prototype
>>   xen: don't include asm/hypercall.h from C sources
>>   add .gitignore entries for *.[is] below xen
>>   xen: generate hypercall interface related code
>>   xen: use generated prototypes for hypercall handlers
>>   x86/pv-shim: don't modify hypercall table
>>   xen/x86: don't use hypercall table for calling compat hypercalls
>>   xen/x86: call hypercall handlers via generated macro
>>   xen/arm: call hypercall handlers via generated macro
>>   xen/x86: add hypercall performance counters for hvm, correct pv
>>   xen: drop calls_to_multicall performance counter
>>   tools/xenperf: update hypercall names
> 
> https://gitlab.com/xen-project/people/andyhhp/xen/-/jobs/1752237172
> 
> Something here causes
> 
> hypercall.c: In function 'hvm_hypercall':
> hypercall.c:174:23: error: unused variable 'r8' [-Werror=unused-variable]
>   174 |         unsigned long r8 = regs->r8;
>       |                       ^~
> hypercall.c:190:22: error: unused variable 'edi' [-Werror=unused-variable]
>   190 |         unsigned int edi = regs->edi;
>       |                      ^~~
> cc1: all warnings being treated as errors
> 
> I suspect it will be "call hypercall handlers via generated macro", but
> I haven't investigated further.

I suspect that's a randconfig build with !HYPFS + !ARGO. The hypfs and
argo hypercalls are the only ones with 5 parameters that HVM wires up.
A similar issue ought to exist in PV hypercall handling though, as the
compat form of update_va_mapping_otherdomain is the only other one
taking 5 arguments.

As to possible solutions - besides maybe adding (void)r8 to the -ENOSYS
path, I was wondering anyway in how far the local variables are still
warranted to retain.

Jan