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Re: [Xen-devel] [PATCH v2 01/20] x86: make hvm_{get/set}_param accessible
- To: Tamas K Lengyel <tamas.k.lengyel@xxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Fri, 20 Dec 2019 17:46:55 +0000
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- Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Fri, 20 Dec 2019 17:47:04 +0000
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On 20/12/2019 17:36, Tamas K Lengyel wrote:
> 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
> https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=tools/libxc/xc_sr_save_x86_hvm.c;h=97a8c49807f192c47209525f51e4d79a50c66cec;hb=HEAD#l61
> 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
> rest.
I think you're trying to ascribe any form of design/plan to a system
which had none. :)
The code you quote was like that because that is how legacy migration
worked. That said, eliding empty records was an effort-saving exercise
(avoid redundant hypercalls on destination side), not because there was
any suggestion that attempting to explicitly set 0 would crash.
Do you have any idea which param was causing problems?
~Andrew
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