[PATCH 9/9] um: Add initial SMP support

[Johannes Berg]

On Sun, 2025-07-27 at 14:29 +0800, Tiwei Bie wrote:
> 
> +++ b/arch/um/include/asm/smp.h
> @@ -2,6 +2,27 @@
>  #ifndef __UM_SMP_H
>  #define __UM_SMP_H
>  
> -#define hard_smp_processor_id()		0
> +#if IS_ENABLED(CONFIG_SMP)
> +
> +#include <linux/bitops.h>
> +#include <asm/current.h>
> +#include <linux/cpumask.h>
> +#include <shared/smp.h>
> +
> +#define raw_smp_processor_id() uml_curr_cpu()
> +
> +void arch_smp_send_reschedule(int cpu);
> +
> +void arch_send_call_function_single_ipi(int cpu);
> +
> +void arch_send_call_function_ipi_mask(const struct cpumask *mask);
> +
> +static inline void smp_cpus_done(unsigned int maxcpus) { }
> +
> +#else /* !CONFIG_SMP */
> +
> +#define raw_smp_processor_id() 0

This seems a bit odd to me, linux/smp.h also defines
raw_smp_processor_id() to 0 the same way, unconditionally.

It almost seems to me we should define raw_smp_processor_id() only for
SMP? But then also __smp_processor_id()? Maybe not?

linux-arch folks, do you have any comments?

> --- /dev/null
> +++ b/arch/um/include/asm/spinlock.h
> @@ -0,0 +1,8 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __ASM_UM_SPINLOCK_H
> +#define __ASM_UM_SPINLOCK_H
> +
> +#include <asm/processor.h>
> +#include <asm-generic/spinlock.h>
> +
> +#endif /* __ASM_UM_SPINLOCK_H */

Do we need this file? Maybe asm-generic should be including the right
things it needs?

> +void enter_turnstile(struct mm_id *mm_id);
> +void exit_turnstile(struct mm_id *mm_id);

We could add __acquires(turnstile) and __releases(turnstile) or
something, to have sparse check that it's locked/unlocked correctly, but
not sure it's worth it.

> +int disable_kmalloc[NR_CPUS] = { 0 };

nit: you can remove the "0".

> +int smp_sigio_handler(struct uml_pt_regs *regs)
> +{
> +	int cpu = raw_smp_processor_id();
> +
> +	IPI_handler(cpu, regs);
> +	if (cpu != 0)
> +		return 1;
> +	return 0;

nit: "return cpu != 0;" perhaps

> +__uml_setup("ncpus=", uml_ncpus_setup,
> +"ncpus=<# of desired CPUs>\n"
> +"    This tells UML how many virtual processors to start. The maximum\n"
> +"    number of supported virtual processors can be obtained by querying\n"
> +"    the CONFIG_NR_CPUS option using --showconfig.\n\n"


I feel like probably this should at least for now be mutually exclusive
with time-travel= parameters, at least if it's not 1? Or perhaps only
with time-travel=ext?

The timer code is in another patch, will look at that also. I guess
until then it's more of a gut feeling on "how would this work" :)

johannes