Re: [PATCH v3 09/14] x86/xen/msr: Remove calling native_{read,write}_msr{,_safe}() in pmu_msr_{read,write}()

["Mi, Dapeng" <dapeng1.mi@xxxxxxxxxxxxxxx>]

On 4/25/2025 4:34 PM, Xin Li (Intel) wrote:
> hpa found that pmu_msr_write() is actually a completely pointless
> function [1]: all it does is shuffle some arguments, then calls
> pmu_msr_chk_emulated() and if it returns true AND the emulated flag
> is clear then does *exactly the same thing* that the calling code
> would have done if pmu_msr_write() itself had returned true.  And
> pmu_msr_read() does the equivalent stupidity.
>
> Remove the calls to native_{read,write}_msr{,_safe}() within
> pmu_msr_{read,write}().  Instead reuse the existing calling code
> that decides whether to call native_{read,write}_msr{,_safe}() based
> on the return value from pmu_msr_{read,write}().  Consequently,
> eliminate the need to pass an error pointer to pmu_msr_{read,write}().
>
> While at it, refactor pmu_msr_write() to take the MSR value as a u64
> argument, replacing the current dual u32 arguments, because the dual
> u32 arguments were only used to call native_write_msr{,_safe}(), which
> has now been removed.
>
> [1]: 
> https://lore.kernel.org/lkml/0ec48b84-d158-47c6-b14c-3563fd14bcc4@xxxxxxxxx/
>
> Suggested-by: H. Peter Anvin (Intel) <hpa@xxxxxxxxx>
> Sign-off-by: Xin Li (Intel) <xin@xxxxxxxxx>
> ---
>
> Change in v3:
> *) Rename pmu_msr_{read,write}() to pmu_msr_{read,write}_emulated()
>    (Dapeng Mi).
> *) Fix a pmu_msr_read() callsite with wrong arguments (Dapeng Mi).
> ---
>  arch/x86/xen/enlighten_pv.c |  8 ++++++--
>  arch/x86/xen/pmu.c          | 27 ++++-----------------------
>  arch/x86/xen/xen-ops.h      |  4 ++--
>  3 files changed, 12 insertions(+), 27 deletions(-)
>
> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
> index 846b5737d320..61e51a970f3c 100644
> --- a/arch/x86/xen/enlighten_pv.c
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -1090,7 +1090,7 @@ static u64 xen_do_read_msr(unsigned int msr, int *err)
>  {
>       u64 val = 0;    /* Avoid uninitialized value for safe variant. */
>  
> -     if (pmu_msr_read(msr, &val, err))
> +     if (pmu_msr_read_emulated(msr, &val))
>               return val;
>  
>       if (err)
> @@ -1132,6 +1132,8 @@ static void set_seg(unsigned int which, unsigned int 
> low, unsigned int high,
>  static void xen_do_write_msr(unsigned int msr, unsigned int low,
>                            unsigned int high, int *err)
>  {
> +     u64 val;
> +
>       switch (msr) {
>       case MSR_FS_BASE:
>               set_seg(SEGBASE_FS, low, high, err);
> @@ -1158,7 +1160,9 @@ static void xen_do_write_msr(unsigned int msr, unsigned 
> int low,
>               break;
>  
>       default:
> -             if (!pmu_msr_write(msr, low, high, err)) {
> +             val = (u64)high << 32 | low;
> +
> +             if (!pmu_msr_write_emulated(msr, val)) {
>                       if (err)
>                               *err = native_write_msr_safe(msr, low, high);
>                       else
> diff --git a/arch/x86/xen/pmu.c b/arch/x86/xen/pmu.c
> index 9c1682af620a..b6557f2d1a2e 100644
> --- a/arch/x86/xen/pmu.c
> +++ b/arch/x86/xen/pmu.c
> @@ -313,37 +313,18 @@ static bool pmu_msr_chk_emulated(unsigned int msr, 
> uint64_t *val, bool is_read,
>       return true;
>  }
>  
> -bool pmu_msr_read(unsigned int msr, uint64_t *val, int *err)
> +bool pmu_msr_read_emulated(u32 msr, u64 *val)
>  {
>       bool emulated;
>  
> -     if (!pmu_msr_chk_emulated(msr, val, true, &emulated))
> -             return false;
> -
> -     if (!emulated) {
> -             *val = err ? native_read_msr_safe(msr, err)
> -                        : native_read_msr(msr);
> -     }
> -
> -     return true;
> +     return pmu_msr_chk_emulated(msr, val, true, &emulated) && emulated;
>  }
>  
> -bool pmu_msr_write(unsigned int msr, uint32_t low, uint32_t high, int *err)
> +bool pmu_msr_write_emulated(u32 msr, u64 val)
>  {
> -     uint64_t val = ((uint64_t)high << 32) | low;
>       bool emulated;
>  
> -     if (!pmu_msr_chk_emulated(msr, &val, false, &emulated))
> -             return false;
> -
> -     if (!emulated) {
> -             if (err)
> -                     *err = native_write_msr_safe(msr, low, high);
> -             else
> -                     native_write_msr(msr, low, high);
> -     }
> -
> -     return true;
> +     return pmu_msr_chk_emulated(msr, &val, false, &emulated) && emulated;
>  }
>  
>  static u64 xen_amd_read_pmc(int counter)
> diff --git a/arch/x86/xen/xen-ops.h b/arch/x86/xen/xen-ops.h
> index 735f58780704..163e03e33089 100644
> --- a/arch/x86/xen/xen-ops.h
> +++ b/arch/x86/xen/xen-ops.h
> @@ -274,8 +274,8 @@ void xen_pmu_finish(int cpu);
>  static inline void xen_pmu_init(int cpu) {}
>  static inline void xen_pmu_finish(int cpu) {}
>  #endif
> -bool pmu_msr_read(unsigned int msr, uint64_t *val, int *err);
> -bool pmu_msr_write(unsigned int msr, uint32_t low, uint32_t high, int *err);
> +bool pmu_msr_read_emulated(u32 msr, u64 *val);
> +bool pmu_msr_write_emulated(u32 msr, u64 val);
>  int pmu_apic_update(uint32_t reg);
>  u64 xen_read_pmc(int counter);
>  

Reviewed-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>