Re: [PATCH RFC V3 4/9] x86/pks: Preserve the PKRS MSR on context switch

[Dave Hansen]

On 10/9/20 12:42 PM, ira.weiny@xxxxxxxxx wrote:
> From: Ira Weiny <ira.weiny@xxxxxxxxx>
> 
> The PKRS MSR is defined as a per-logical-processor register.  This
> isolates memory access by logical CPU.  Unfortunately, the MSR is not
> managed by XSAVE.  Therefore, tasks must save/restore the MSR value on
> context switch.
> 
> Define a saved PKRS value in the task struct, as well as a cached
> per-logical-processor MSR value which mirrors the MSR value of the
> current CPU.  Initialize all tasks with the default MSR value.  Then, on
> schedule in, check the saved task MSR vs the per-cpu value.  If
> different proceed to write the MSR.  If not avoid the overhead of the
> MSR write and continue.

It's probably nice to note how the WRMSR is special here, in addition to
the comments below.

>  #endif /*_ASM_X86_PKEYS_INTERNAL_H */
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index 97143d87994c..da2381136b2d 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -18,6 +18,7 @@ struct vm86;
>  #include <asm/cpufeatures.h>
>  #include <asm/page.h>
>  #include <asm/pgtable_types.h>
> +#include <asm/pkeys_common.h>
>  #include <asm/percpu.h>
>  #include <asm/msr.h>
>  #include <asm/desc_defs.h>
> @@ -542,6 +543,11 @@ struct thread_struct {
>  
>  	unsigned int		sig_on_uaccess_err:1;
>  
> +#ifdef	CONFIG_ARCH_HAS_SUPERVISOR_PKEYS
> +	/* Saved Protection key register for supervisor mappings */
> +	u32			saved_pkrs;
> +#endif

Could you take a look around thread_struct and see if there are some
other MSRs near which you can stash this?  This seems like a bit of a
lonely place.

...
>  void flush_thread(void)
>  {
>  	struct task_struct *tsk = current;
> @@ -195,6 +212,8 @@ void flush_thread(void)
>  	memset(tsk->thread.tls_array, 0, sizeof(tsk->thread.tls_array));
>  
>  	fpu__clear_all(&tsk->thread.fpu);
> +
> +	pks_init_task(tsk);
>  }
>  
>  void disable_TSC(void)
> @@ -644,6 +663,8 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p)
>  
>  	if ((tifp ^ tifn) & _TIF_SLD)
>  		switch_to_sld(tifn);
> +
> +	pks_sched_in();
>  }
>  
>  /*
> diff --git a/arch/x86/mm/pkeys.c b/arch/x86/mm/pkeys.c
> index 3cf8f775f36d..30f65dd3d0c5 100644
> --- a/arch/x86/mm/pkeys.c
> +++ b/arch/x86/mm/pkeys.c
> @@ -229,3 +229,31 @@ u32 update_pkey_val(u32 pk_reg, int pkey, unsigned int flags)
>  
>  	return pk_reg;
>  }
> +
> +DEFINE_PER_CPU(u32, pkrs_cache);
> +
> +/**
> + * It should also be noted that the underlying WRMSR(MSR_IA32_PKRS) is not
> + * serializing but still maintains ordering properties similar to WRPKRU.
> + * The current SDM section on PKRS needs updating but should be the same as
> + * that of WRPKRU.  So to quote from the WRPKRU text:
> + *
> + * 	WRPKRU will never execute transiently. Memory accesses
> + * 	affected by PKRU register will not execute (even transiently)
> + * 	until all prior executions of WRPKRU have completed execution
> + * 	and updated the PKRU register.
> + */
> +void write_pkrs(u32 new_pkrs)
> +{
> +	u32 *pkrs;
> +
> +	if (!static_cpu_has(X86_FEATURE_PKS))
> +		return;
> +
> +	pkrs = get_cpu_ptr(&pkrs_cache);
> +	if (*pkrs != new_pkrs) {
> +		*pkrs = new_pkrs;
> +		wrmsrl(MSR_IA32_PKRS, new_pkrs);
> +	}
> +	put_cpu_ptr(pkrs);
> +}
> 

It bugs me a *bit* that this is being called in a preempt-disabled
region, but we still bother with the get/put_cpu jazz.  Are there other
future call-sites for this that aren't in preempt-disabled regions?