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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.
>
> Jan
>

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);

Which changes the behaviour of rtc_mode_is(s, mode) in Xen.

The presence (or lack thereof) of the WAET table does not address the problem 
at hand, which is that Xen's current logic of trying to guess what the guest is 
doing WRT RTC is wrong. It causes the guest to fall into an infinite loop 
expecting standards compliment behaviour, where Xen has decided that the guest 
has moved to the optimised behaviour.

~Andrew


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