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

[Boris Ostrovsky]

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()?

-boris