Re: [Xen-devel] [PATCH v3 4/4] x86/PV: enable the emulated PIT

[Roger Pau Monnà <roger.pau@xxxxxxxxxx>]

El 18/01/16 a les 11.41, Andrew Cooper ha escrit:
> On 18/01/16 09:44, Jan Beulich wrote:
>>>>> On 18.01.16 at 10:29, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> On 18/01/2016 07:43, Jan Beulich wrote:
>>>>>>> On 15.01.16 at 18:45, <roger.pau@xxxxxxxxxx> wrote:
>>>>> Changes since v2:
>>>>>  - Change 'if ( (a && b) || (!a && c) )' into 'if ( a ? b : c )'.
>>>> Thanks, but after some more thinking about it I'm afraid there are
>>>> a few more aspects to consider here:
>>>>
>>>>> --- a/xen/arch/x86/domain.c
>>>>> +++ b/xen/arch/x86/domain.c
>>>>> @@ -542,8 +542,9 @@ int arch_domain_create(struct domain *d, unsigned int 
>>> domcr_flags,
>>>>>                     d->domain_id, config->emulation_flags);
>>>>>              return -EINVAL;
>>>>>          }
>>>>> -        if ( config->emulation_flags != 0 &&
>>>>> -             (!is_hvm_domain(d) || config->emulation_flags != 
>>>>> XEN_X86_EMU_ALL) 
>>> )
>>>>> +        if ( is_hvm_domain(d) ? (config->emulation_flags != 
>>>>> XEN_X86_EMU_ALL &&
>>>>> +             config->emulation_flags != 0) :
>>>>> +             (config->emulation_flags != XEN_X86_EMU_PIT) )
>>>>>          {
>>>> For one I think it would be a good idea to allow zero for PV domains,
>>>> and perhaps even default new DomU-s to have the PIT flag clear.
>>>> (Also - indentation.)
>>>>
>>>> Which gets us to the second, broader issue: These flags shouldn't
>>>> be forced to a particular value during migration, but instead they
>>>> should be part of the state getting migrated. Incoming domains
>>>> then would - if the field is missing due to coming from an older
>>>> hypervisor - have the flag default to 1.
>>> There is sadly another ratsnest here.
>> I've been afraid of that.
>>
>>> These values are needed for domain creation, which means that putting
>>> them anywhere in the migration stream is already too late, as the domain
>>> has been created before the stream header is read.
>> Is that an inherent requirement, or just a result of current code
>> structure?
> 
> Depends.  As far as libxc/libxl migration levels go, current code structure.
> 
> Whatever (eventually) gets used to set these values will however be
> present in the xl configuration, which is at the very start of the
> stream, and is what is used to create the new domain.
> 
> We really don't want the libxc migrate code to be making the
> DOMCTL_createdomain hypercall itself; it opens up a whole new attack
> surface via cunningly-crafted save image.  The best we can do is have a
> sanity check later on.
> 
>>  I ask because migrating the emulation flags is going to
>> be a requirement for relaxing the current (almost) all-or-nothing
>> policy on those flags.
>>
>>> In principle, the best which could occur is that a value gets stashed in
>>> the stream and used as a sanity check.  That will at least catch the
>>> case when they are different.
>> That'd be a minimal first step.
> 
> This is a substantial quantity of work to do properly.  As the emulation
> flags are just one in a very long list of fields handed like this, I
> don't think this issue should block the series.

You certainly are more familiar with the migration code than me, but
wouldn't it be enough to add a new field to libxl_domain_build_info
(uint32_t emulation_flags), and teach
libxl_domain_build_info_gen_json/libxl__domain_build_info_parse_json
 how to properly parse it?

This however raises the question about how to signal that the field is
not initialised, because 0 is a valid value (maybe ~0)?

Roger.


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