From: David Gwynne <david@gwynne.id.au>
Subject: Re: cpu_xcall glue for amd64
To: Mike Larkin <mlarkin@nested.page>
Cc: OpenBSD Tech <tech@openbsd.org>
Date: Sun, 13 Jul 2025 19:13:11 +1000



> On 13 Jul 2025, at 17:33, Mike Larkin <mlarkin@nested.page> wrote:
> 
> On Sun, Jul 13, 2025 at 03:56:45PM +1000, David Gwynne wrote:
>> if a driver needs cpu_xcall, it can depend on xcall and enable
>> compilation of the xcall code and this glue.
>> 
>> ok?
>> 
> 
> If we're going to do xcalls at all, why even bother making it conditional?

it has to be conditional on which archs implement the MD glue at least.

> 
> Eg, I don't see any benefit of having some kernels be non-xcall and some
> xcall enabled based on what drivers are in them.

we just went through an effort to shrink kernel code to help the install media fit. it seems a bit ungrateful to use those bytes again for something that's generally only going to be used for performance monitoring.

plus, ive already done the work.

> 
> -ml
> 
>> Index: amd64/cpu.c
>> ===================================================================
>> RCS file: /cvs/src/sys/arch/amd64/amd64/cpu.c,v
>> diff -u -p -r1.196 cpu.c
>> --- amd64/cpu.c 5 Jun 2025 09:29:54 -0000 1.196
>> +++ amd64/cpu.c 13 Jul 2025 05:50:03 -0000
>> @@ -637,6 +637,10 @@ cpu_attach(struct device *parent, struct
>> #endif
>> 
>> #if defined(MULTIPROCESSOR)
>> +#if NXCALL > 0
>> + cpu_xcall_establish(ci);
>> +#endif
>> +
>> /*
>> * Allocate UPAGES contiguous pages for the idle PCB and stack.
>> */
>> Index: amd64/ipifuncs.c
>> ===================================================================
>> RCS file: /cvs/src/sys/arch/amd64/amd64/ipifuncs.c,v
>> diff -u -p -r1.39 ipifuncs.c
>> --- amd64/ipifuncs.c 7 Jun 2024 16:53:35 -0000 1.39
>> +++ amd64/ipifuncs.c 13 Jul 2025 05:50:03 -0000
>> @@ -108,6 +108,11 @@ void (*ipifunc[X86_NIPI])(struct cpu_inf
>> NULL,
>> #endif
>> x86_64_ipi_wbinvd,
>> +#if NXCALL > 0
>> + cpu_xcall_dispatch,
>> +#else
>> + NULL,
>> +#endif
>> };
>> 
>> void
>> Index: include/cpu.h
>> ===================================================================
>> RCS file: /cvs/src/sys/arch/amd64/include/cpu.h,v
>> diff -u -p -r1.180 cpu.h
>> --- include/cpu.h 28 Apr 2025 16:18:25 -0000 1.180
>> +++ include/cpu.h 13 Jul 2025 05:50:04 -0000
>> @@ -42,9 +42,12 @@
>>  * Definitions unique to x86-64 cpu support.
>>  */
>> #ifdef _KERNEL
>> +#include "xcall.h"
>> +
>> #include <machine/frame.h>
>> #include <machine/segments.h> /* USERMODE */
>> #include <machine/intrdefs.h>
>> +#include <sys/xcall.h>
>> #endif /* _KERNEL */
>> 
>> #include <sys/clockintr.h>
>> @@ -215,6 +218,9 @@ struct cpu_info {
>> 
>> #ifdef MULTIPROCESSOR
>> struct srp_hazard ci_srp_hazards[SRP_HAZARD_NUM];
>> +#if NXCALL > 0
>> + struct xcall_cpu ci_xcall;
>> +#endif
>> #define __HAVE_UVM_PERCPU
>> struct uvm_pmr_cache ci_uvm; /* [o] page cache */
>> #endif
>> Index: include/intr.h
>> ===================================================================
>> RCS file: /cvs/src/sys/arch/amd64/include/intr.h,v
>> diff -u -p -r1.36 intr.h
>> --- include/intr.h 11 Jun 2025 09:57:01 -0000 1.36
>> +++ include/intr.h 13 Jul 2025 05:50:04 -0000
>> @@ -222,7 +222,9 @@ void x86_ipi_handler(void);
>> void x86_setperf_ipi(struct cpu_info *);
>> 
>> extern void (*ipifunc[X86_NIPI])(struct cpu_info *);
>> -#endif
>> +
>> +#define cpu_xcall_ipi(_ci) x86_send_ipi((_ci), X86_IPI_XCALL)
>> +#endif /* MULTIPROCESSOR */
>> 
>> #endif /* !_LOCORE */
>> 
>> Index: include/intrdefs.h
>> ===================================================================
>> RCS file: /cvs/src/sys/arch/amd64/include/intrdefs.h,v
>> diff -u -p -r1.24 intrdefs.h
>> --- include/intrdefs.h 26 May 2024 13:37:31 -0000 1.24
>> +++ include/intrdefs.h 13 Jul 2025 05:50:04 -0000
>> @@ -85,8 +85,9 @@
>> #define X86_IPI_START_VMM 0x00000100
>> #define X86_IPI_STOP_VMM 0x00000200
>> #define X86_IPI_WBINVD 0x00000400
>> +#define X86_IPI_XCALL 0x00000800
>> 
>> -#define X86_NIPI 12
>> +#define X86_NIPI 13
>> 
>> #define IREENT_MAGIC 0x18041969
>> 
>>