Re: [Xen-devel] [PATCH 11/13] x86/paravirt: Add paravirt alternatives infrastructure

[Josh Poimboeuf]

On Tue, Oct 17, 2017 at 04:59:41PM -0400, Boris Ostrovsky wrote:
> On 10/17/2017 04:50 PM, Josh Poimboeuf wrote:
> > On Tue, Oct 17, 2017 at 04:36:00PM -0400, Boris Ostrovsky wrote:
> >> On 10/17/2017 04:17 PM, Josh Poimboeuf wrote:
> >>> On Tue, Oct 17, 2017 at 11:36:57AM -0400, Boris Ostrovsky wrote:
> >>>> On 10/17/2017 10:36 AM, Josh Poimboeuf wrote:
> >>>>> Maybe we can add a new field to the alternatives entry struct which
> >>>>> specifies the offset to the CALL instruction, so apply_alternatives()
> >>>>> can find it.
> >>>> We'd also have to assume that the restore part of an alternative entry
> >>>> is the same size as the save part. Which is true now.
> >>> Why?
> >>>
> >> Don't you need to know the size of the instruction without save and
> >> restore part?
> >>
> >> + if (a->replacementlen == 6 && *insnbuf == 0xff && *(insnbuf+1) == 0x15)
> >>
> >> Otherwise you'd need another field for the actual instruction length.
> > If we know where the CALL instruction starts, and can verify that it
> > starts with "ff 15", then we know the instruction length: 6 bytes.
> > Right?
> >
> 
> Oh, OK. Then you shouldn't need a->replacementlen test(s?) in
> apply_alternatives()?

Right.  Though in the above code it was needed for a different reason,
to make sure it wasn't reading past the end of the buffer.

-- 
Josh