Re: [PATCH v2 1/4] xl: Add support for ignore_msrs option

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.02.2021 11:42, Roger Pau Monné wrote:
> On Wed, Jan 20, 2021 at 05:49:09PM -0500, Boris Ostrovsky wrote:
>> This option allows guest administrator specify what should happen when
>> guest accesses an MSR which is not explicitly emulated by the hypervisor.
>>
>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
>> ---
>>  docs/man/xl.cfg.5.pod.in         | 20 +++++++++++++++++++-
>>  tools/libs/light/libxl_types.idl |  7 +++++++
>>  tools/xl/xl_parse.c              |  7 +++++++
>>  3 files changed, 33 insertions(+), 1 deletion(-)
>>
>> diff --git a/docs/man/xl.cfg.5.pod.in b/docs/man/xl.cfg.5.pod.in
>> index c8e017f950de..96ce97c42cab 100644
>> --- a/docs/man/xl.cfg.5.pod.in
>> +++ b/docs/man/xl.cfg.5.pod.in
>> @@ -2044,7 +2044,25 @@ Do not provide a VM generation ID.
>>  See also "Virtual Machine Generation ID" by Microsoft:
>>  
>> L<https://docs.microsoft.com/en-us/windows/win32/hyperv_v2/virtual-machine-generation-identifier>
>>  
>> -=back 
>> +=over
>> +
>> +=item B<ignore_msrs="STRING">
>> +
>> +Determine hypervisor behavior on accesses to MSRs that are not emulated by 
>> the hypervisor.
>> +
>> +=over 4
>> +
>> +=item B<never>
>> +
>> +Issue a warning to the log and #GP to the guest. This is default.
>> +
>> +=item B<silent>
>> +
>> +MSR reads return 0, MSR writes are ignored. No warnings to the log.
>> +
>> +=item B<verbose>
>> +
>> +Similar to B<silent> but a warning is written.
> 
> Would it make sense to allow for this option to be more fine-grained
> in the future?

>From an abstract perspective - maybe. But remember that this information
will need to be migrated with the guest. It would seem to me that
Boris'es approach is easier migration-wise.

> Not that you need to implement the full thing now, but maybe we could
> have something like:
> 
> "
> =item B<ignore_msrs=[ "MSR_RANGE, "MSR_RANGE", ..]>
> 
> Specify a list of MSR ranges that will be ignored by the hypervisor:
> reads will return zeros and writes will be discarded without raising a
> #GP.
> 
> Each MSR_RANGE is given in hexadecimal format and may be a range, e.g.
> c00102f0-c00102f1 (inclusive), or a single MSR, e.g. c00102f1.
> "
> 
> Then you can print the messages in the hypervisor using a guest log
> level and modify it on demand in order to get more verbose output?

"Modify on demand"? Irrespective of what you mean with this, ...

> I don't think selecting whether the messages are printed or not from
> xl is that helpful as the same could be achieved using guest_loglvl.

... controlling this via guest_loglvl would affect various other
log messages' visibility.

Jan