Re: [PATCH resend v3] hw/i386: pass RNG seed via setup_data entry

[Jason A. Donenfeld]

Hey again,

On Thu, Jul 21, 2022 at 11:47:29AM +0200, Jason A. Donenfeld wrote:
> Hi Paolo,
> 
> Thanks for your review.
> 
> On Thu, Jul 21, 2022 at 11:19:40AM +0200, Paolo Bonzini wrote:
> > The issue with live migration is that the setup data changes from before 
> > to after the patches.  This means that a live migration exactly _in the 
> > middle_ of reading the Linux boot data could fail badly.  For example, 
> > you could migrate in the middle of reading the DTB, and it would be 
> > shifted by the ~50 bytes of the setup_data and seed.  The size would 
> > also not match so, even though probably things would mostly work if you 
> > place the seed last, that's not really optimal either.
> 
> This doesn't really make sense to me, as I don't think the machine can
> even be migrated during x86_load_linux(), and a migration will skip this
> whole step anyway since this is mutable memory that a live kernel does
> mutate.
> 
> However, what I'll do is reverse the order of these, so that the DTB is
> added first, and I'll only set up the links in the right order so that
> there's no potential race. I'll send a v+1 doing this shortly.

As I implement the "race-free" version, I notice that this is even more
of a non-issue, seeing as even without this patch, the DTB is loaded
after the length is written. What you're talking about is just not real.
I'll still send a v+1 changing the order, because that seems better
anyway, but the race thing seems pretty imaginary...

Jason