lkml.org 
[lkml]   [2020]   [Oct]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH RFC V3 4/9] x86/pks: Preserve the PKRS MSR on context switch
From
Date
On 10/9/20 12:42 PM, ira.weiny@intel.com wrote:
> From: Ira Weiny <ira.weiny@intel.com>
>
> 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?

\
 
 \ /
  Last update: 2020-10-13 20:32    [W:0.339 / U:0.788 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site