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Subject: Re: [Xen-devel] [PATCH 11/13] x86/paravirt: Add paravirt alternatives
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To: Brian Gerst <brgerst@gmail.com>
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From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Date: Tue, 17 Oct 2017 10:05:47 -0400
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On 10/17/2017 09:10 AM, Brian Gerst wrote:
> On Mon, Oct 16, 2017 at 2:18 PM, Boris Ostrovsky
> <boris.ostrovsky@oracle.com> wrote:
>>
>> Replacing the macro with
>>
>> #define PV_INDIRECT(addr)       *addr  // well, it's not so much
>> indirect anymore
>>
>> makes things work. Or maybe it can be adjusted top be kept truly indirect.
> That is still an indirect call, just using absolute addressing for the
> pointer instead of RIP-relative. 

Oh, right, I've got my terminology all wrong.

-boris

>  Alternatives has very limited
> relocation capabilities.  It will only handle a single call or jmp
> replacement. Using absolute addressing is slightly less efficient
> (takes one extra byte to encode, and needs a relocation for KASLR),
> but it works just as well.  You could also relocate the instruction
> manually by adding the delta between the original and replacement code
> to the displacement.