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

[Juergen Gross <jgross@xxxxxxxx>]

On 08.11.21 09:36, Jan Beulich wrote:
> 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.
I think dropping those local variables is the easiest solution. And I
think this applies to the PV variant, too.


Juergen

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