Re: [Xen-devel] [PATCH 3/9] x86/vmx: Factor locate_msr_entry() out of vmx_find_msr() and vmx_add_msr()

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> Sent: Tuesday, May 22, 2018 7:21 PM
> 
> Instead of having multiple algorithms searching the MSR lists, implement a
> single one.  It has the semantics required by vmx_add_msr(), to identify the
> position in which an MSR should live, if it isn't already present.
> 
> There will be a marginal improvement for vmx_find_msr() by avoiding the
> function pointer calls to vmx_msr_entry_key_cmp(), and a major
> improvement for
> vmx_add_msr() by using a binary search instead of a linear search.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Acked-by: Kevin Tian <kevin.tian@xxxxxxxxx>, with one typo found:

> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Jun Nakajima <jun.nakajima@xxxxxxxxx>
> CC: Kevin Tian <kevin.tian@xxxxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
>  xen/arch/x86/hvm/vmx/vmcs.c | 42 ++++++++++++++++++++++++++++-----
> ---------
>  1 file changed, 28 insertions(+), 14 deletions(-)
> 
> diff --git a/xen/arch/x86/hvm/vmx/vmcs.c
> b/xen/arch/x86/hvm/vmx/vmcs.c
> index f557857..e4acdc1 100644
> --- a/xen/arch/x86/hvm/vmx/vmcs.c
> +++ b/xen/arch/x86/hvm/vmx/vmcs.c
> @@ -1276,24 +1276,36 @@ static int construct_vmcs(struct vcpu *v)
>      return 0;
>  }
> 
> -static int vmx_msr_entry_key_cmp(const void *key, const void *elt)
> +/*
> + * Search an MSR list looking for an MSR entry, or the slot in which it
> should
> + * live (to keep the data sorted) if an entry is not found.
> + *
> + * The return pointer is guarenteed to be bounded by start and end.

guaranteed

> However,
> + * it may point at end, and may be invalid for the caller to dereference.
> + */
> +static struct vmx_msr_entry *locate_msr_entry(
> +    struct vmx_msr_entry *start, struct vmx_msr_entry *end, uint32_t msr)
>  {
> -    const u32 *msr = key;
> -    const struct vmx_msr_entry *entry = elt;
> +    while ( start < end )
> +    {
> +        struct vmx_msr_entry *mid = start + (end - start) / 2;
> 
> -    if ( *msr > entry->index )
> -        return 1;
> -    if ( *msr < entry->index )
> -        return -1;
> +        if ( msr < mid->index )
> +            end = mid;
> +        else if ( msr > mid->index )
> +            start = mid + 1;
> +        else
> +            return mid;
> +    }
> 
> -    return 0;
> +    return start;
>  }
> 
>  struct vmx_msr_entry *vmx_find_msr(uint32_t msr, enum
> vmx_msr_list_type type)
>  {
>      struct vcpu *curr = current;
>      struct arch_vmx_struct *arch_vmx = &curr->arch.hvm_vmx;
> -    struct vmx_msr_entry *start = NULL;
> +    struct vmx_msr_entry *start = NULL, *ent, *end;
>      unsigned int total;
> 
>      switch ( type )
> @@ -1315,8 +1327,10 @@ struct vmx_msr_entry *vmx_find_msr(uint32_t
> msr, enum vmx_msr_list_type type)
>      if ( !start )
>          return NULL;
> 
> -    return bsearch(&msr, start, total, sizeof(struct vmx_msr_entry),
> -                   vmx_msr_entry_key_cmp);
> +    end = start + total;
> +    ent = locate_msr_entry(start, end, msr);
> +
> +    return ((ent < end) && (ent->index == msr)) ? ent : NULL;
>  }
> 
>  int vmx_add_msr(uint32_t msr, enum vmx_msr_list_type type)
> @@ -1368,10 +1382,10 @@ int vmx_add_msr(uint32_t msr, enum
> vmx_msr_list_type type)
> 
>      start = *ptr;
>      end   = start + total;
> +    ent   = locate_msr_entry(start, end, msr);
> 
> -    for ( ent = start; ent < end && ent->index <= msr; ++ent )
> -        if ( ent->index == msr )
> -            return 0;
> +    if ( (ent < end) && (ent->index == msr) )
> +        return 0;
> 
>      if ( total == (PAGE_SIZE / sizeof(*ent)) )
>          return -ENOSPC;
> --
> 2.1.4

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