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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 23/05/18 17:39, Roger Pau Monné wrote:
> On Tue, May 22, 2018 at 12:20:40PM +0100, Andrew Cooper wrote:
>> 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>
>> ---
>> 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.  
>> 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;
>> +    }
> This is basically an open coded version of bsearch, isn't there anyway
> to adapt the current bsearch so that it could be used for both
> vmx_find_msr and vmx_add_msr?
>
> I know there will be a performance penalty for using a function
> pointer for the comparator function, but this looks like code
> duplication to me.

A third use appears in a later patch.  bsearch() doesn't have the
described property on a miss, which is necessary to maintain the lists.

~Andrew

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