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Re: [Xen-devel] [PATCH v2 01/20] x86: make hvm_{get/set}_param accessible

On Fri, Dec 20, 2019 at 10:32 AM Andrew Cooper
<andrew.cooper3@xxxxxxxxxx> wrote:
> On 20/12/2019 17:27, Tamas K Lengyel wrote:
> > On Fri, Dec 20, 2019 at 9:47 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
> >> On 18.12.2019 20:40, Tamas K Lengyel wrote:
> >>> Currently the hvm parameters are only accessible via the HVMOP 
> >>> hypercalls. By
> >>> exposing hvm_{get/set}_param it will be possible for VM forking to copy 
> >>> the
> >>> parameters directly into the clone domain.
> >> Having peeked ahead at patch 17, where this gets used, I wonder why
> >> you want a pair of one-by-one functions, rather than a copy-all one.
> >> This then wouldn't require exposure of the functions you touch here.
> > Well, provided there is no such function in existence today it was
> > just easier to use what's already available. I still wouldn't want to
> > implement a one-shot function like that because this same code-path is
> > shared by the save-restore operations on the toolstack side, so at
> > least I have a reasonable assumption that it won't break on me in the
> > future.
> In particular, a number of the set operations are distinctly
> non-trivial.  (OTOH, those are not long for this world, and should be
> creation X86_EMU_* constants instead).

I actually wanted to ask about that. In
the toolstack only selects certain HVM params to be saved (and
restored later). I originally followed the same logic in the fork
code-path but after a lot of experiments it looks like it's actually
OK to grab all params but only call set_param on the ones that have a
non-zero value. So setting some params with a zero value has certainly
lead to crashes, but otherwise it seems to "just work" to copy all the


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