Re: [Xen-devel] [PATCH 2/2] hvmloader: Allow the toolstack to choose WAET optimisations

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 27.11.13 at 13:14, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> On 27/11/13 10:03, Jan Beulich wrote:
>>>>> On 26.11.13 at 21:39, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>>> @@ -196,8 +201,35 @@ static struct acpi_20_waet *construct_waet(void)
>>>      memcpy(waet, &Waet, sizeof(*waet));
>>>  
>>>      waet->header.length = sizeof(*waet);
>>> +
>>> +    s = xenstore_read("platform/waet-rtc-noack", NULL);
>>> +    if ( s )
>>> +    {
>>> +        if ( !strncmp(s, "1", 1) || !strncmp(s, "true", 4) )
>>> +            waet->flags |= ACPI_WAET_RTC_NO_ACK;
>>> +        else
>>> +            waet->flags &= ~ACPI_WAET_RTC_NO_ACK;
>>> +    }
>>> +
>>> +    s = xenstore_read("platform/waet-pm-reliable", NULL);
>>> +    if ( s )
>>> +    {
>>> +        if ( !strncmp(s, "1", 1) || !strncmp(s, "true", 4) )
>>> +            waet->flags |= ACPI_WAET_TIMER_ONE_READ;
>>> +        else
>>> +            waet->flags &= ~ACPI_WAET_TIMER_ONE_READ;
>>> +    }
>> Nothing in this series really allows these to be easily controlled on a
>> per-domain basis - I would have expected a config file flag...
>>
>> And for this second one, as hinted at in your overview mail, I
>> don't really see what purpose the controlling here serves: You
>> don't consume the setting, i.e. the sole effect is that of
>> controlling the ACPI table's flag value. Yet if we're fine with
>> the guest doing single reads (of whatever), then we're surely
>> fine too with it doing double reads. And hence the flag can
>> easily be always set (as it was till now).
>>
>> If any second override was to be considered, I'd recommend
>> on controlling the presence of the WAET as a whole.
> 
> The setting is consumed using
> 
> +    /* Inform Xen which RTC mode has been chosen */
> +    p.value = !!(waet->flags & ACPI_WAET_RTC_NO_ACK);
> +    hypercall_hvm_op(HVMOP_set_param, &p);

Note that I said "this second one", i.e. referring to
ACPI_WAET_TIMER_ONE_READ.

Jan


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