Re: [Xen-devel] [PATCH v5 9/9] xen/x86: setup PVHv2 Dom0 ACPI tables

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 08.02.17 at 16:58, <roger.pau@xxxxxxxxxx> wrote:
> On Wed, Feb 08, 2017 at 08:50:54AM -0700, Jan Beulich wrote:
>> >>> On 08.02.17 at 16:10, <roger.pau@xxxxxxxxxx> wrote:
>> > On Thu, Jan 26, 2017 at 07:16:32AM -0700, Jan Beulich wrote:
>> >> >>> On 19.01.17 at 18:29, <roger.pau@xxxxxxxxxx> wrote:
>> >> > +    /*
>> >> > +     * The APIC ID field of the local APIC struct is only 1byte, so we 
>> >> > need
>> >> > +     * to limit the number of local APIC entries to 128 because we 
>> >> > only use
>> >> > +     * even numbers as APIC IDs.
>> >> > +     */
>> >> > +    size += sizeof(*local_apic) *
>> >> > +            min(max_vcpus, 1U << (sizeof(local_apic->id) * 8));
>> >> 
>> >> This caps at 256 now. Perhaps it would anyway be better to use
>> >> HVM_MAX_VCPUS here, or maybe the minimum of both?
>> > 
>> > max_vcpus is already capped to HVM_MAX_VCPUS.
>> 
>> Is it? I see a use of that symbol only in mk_dsdt.c.
> 
> It's introduced in a patch of this series, see "x86/PVHv2: fix 
> dom0_max_vcpus
> so it's capped to 128 for PVHv2 Dom0", that's why you don't find it in your
> tree :).

Okay. A little bit disconnected, but anyway.

>> > IMHO it's not right to use
>> > HVM_MAX_VCPUS because we will increase it at some point, and then we would 
>> > add
>> > more local APIC entries that possible thus overflowing the id field.
>> 
>> Hence the "or maybe the minimum of both".
>> 
>> >> Plus, when you cap LAPIC entries, you should also provide x2APIC
>> >> ones (to be able to represent all vCPU-s).
>> > 
>> > Would it make sense to then provide x2APIC entries regardless of whether 
>> > they
>> > are present on the native tables or not?
>> > 
>> > The emulated APIC provided by Xen supports x2APIC mode anyway.
>> 
>> Yes, I think so.
> 
> As a reply to both comments above, I will switch to simply using x2APIC 
> entries
> for all vCPUs, and get rid of all this, so we will only provide x2APIC for
> vCPUs.  Then if HVM_MAX_VCPUS is increased we will have no issue.

If all guests we care about are fine with this - why not.

Jan


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