Re: [Xen-devel] [RFC v2] xSplice design

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 05/06/15 16:00, Konrad Rzeszutek Wilk wrote:
>> As you discussed, if you allocate hotpatch memory withing +-2GB of the
>> > patch location, no further trampoline indirection is required, a
>> > 5-byte JMP does the trick on x86.  We found that to be sufficient in
>> > our experiments.
> And worst case if you do need more than +-2GB you can just have
> two jumps. Kind of silly but possible.
>
> Thank you for your input and lookign forward to your reply!

The Xen combined text/rodata/data/init/bss size is currently less than
4MB.  The alignment for superpages bumps this to 12MB (so plenty of free
space), and arbitrary extra space can be inserted if wanted.  Xen has
(just less than) 1GB of virtual space reserved for this area.

If a hotpatch is going to blow these limits, you have bigger problems to
worry about.

~Andrew

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