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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, May 23, 2018 at 05:55:50PM +0100, Andrew Cooper wrote:
> 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.

I would consider adding a flag to the list of parameters so that
bsearch returned the position where the item should be added in case
of a miss. You could then wrap it inside of locate_msr_entry, or get
rid of this helper altogether.

Thanks, Roger.

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