Re: [Xen-devel] [PATCH v4 6/6] x86/HVM: report the set of enabled emulated devices through CPUID

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

El 22/01/16 a les 11.57, Jan Beulich ha escrit:
>>>> On 21.01.16 at 17:51, <roger.pau@xxxxxxxxxx> wrote:
>> Add a new HVM-specific feature flag that signals the presence of a bitmap
>> that contains the current set of enabled emulated devices. The bitmap is
>> placed in the ecx register. The bit fields used in the bitmap are the same
>> as the ones used in the xen_arch_domainconfig emulation_flags field, and
>> their meaning can be found at arch-x86/xen.h.
>>
>> This will allow Xen to enable emulated devices for HVMlite guests in the
>> future, by having a proper ABI for reporting which devices are enabled.
> 
> The idea is certainly nice and appreciated, but ...
> 
>> --- a/xen/include/public/arch-x86/cpuid.h
>> +++ b/xen/include/public/arch-x86/cpuid.h
>> @@ -78,12 +78,17 @@
>>   * HVM-specific features
>>   * EAX: Features
>>   * EBX: vcpu id (iff EAX has XEN_HVM_CPUID_VCPU_ID_PRESENT flag)
>> + * ECX: bitmap of enabled devices, according to the bit fields defined in
>> + *      arch-x86/xen.h.
> 
> ... this set of definitions is not currently a stable ABI (limited to
> hypervisor and tool stack), and if we wanted to make it stable
> we'd first need to think a little about the complications that may
> arise if the granularity chosen (think about the PM bit and the
> discussion around it before your changes went in) turns out to
> be a problem later on.

Yes, in fact I'm having second thoughts on the PM flag, and I think I
should have split it into ACPI_PM and ACPI_TIMER instead.

> Also at least some of the features can be determined by other
> means (CPUID, ACPI tables), so I'm not even sure we need all
> of this, and I'd really prefer to avoid multiple distinct ways to
> learn of a certain feature, as it's too easy for the two (or more)
> mechanisms to get out of sync.

So let's look at the flags and whether there's an existing way to signal
it's presence:

LAPIC: CPUID.01h:EDX[bit 9]
IOAPIC: tied to LAPIC (so either both enabled or none).

HPET: can only be enabled from/with ACPI, since it's base memory address
is not fixed, and we would need to find a way to pass it's address to
the OS in the absence of ACPI.

RTC: I don't know of any way to signal the RTC presence, AFAICT it's
always assumed to be there in the PC architecture. Could maybe return ~0
when reading from IO port 0x71, but that's meh..., not the best way IMHO.

PIC: same as RTC, I don't know of any way to signal it's presence since
it's assumed to be there.

VGA: again I don't think there's an easy way to signal it's presence,
apart from returning ~0 from the multiple IO ports it uses. The fact
that the 0xA0000-0xBFFFF memory range is also marked as RAM in the e820
map in HVMlite DomUs should also trigger OSes into disabling VGA due to
the lack of proper MMIO range, but sadly I think most OSes just assume
it's there.

PIT: assumed to be always present in the PC architecture.

PM: I'm leaning to split this into ACPI_PM and ACPI_TIMER as said
before. ACPI_TIMER presence it's contained inside of ACPI tables, and
the availability of ACPI_PM (power management) can be inferred from the
presence of ACPI itself.

AMD guest IOMMU: AFAICT this seems to be currently disabled, since the
MMIO range it checks is [~0ULL, ~0ULL + 0x8000]. There is a function to
change the base address ~0ULL to something else, but it doesn't seem to
be reachable from any path. In any case, I guess the presence of this
device will be reported from ACPI.

So, we have the following devices that are assumed to be there: RTC,
PIC, PIT. Everything else I think can be signalled by other means
already available.

IMHO, I think we could say that the PIC is never going to be available
to HVMlite guests (in any case we would enable the lapic/ioapic), and
maybe enable the RTC and PIT by default?

Then I think we could get away without any Xen-specific way of reporting
enabled devices.

>> All unused bits have undefined values.
> 
> Nor is this an option, but maybe this is just a wording issue:
> Perhaps you mean to say that they're reserved for future use?
> Since truly unused bits have are guaranteed to have the value
> zero, just that the set of bits varies.

Yes, that's exactly what I meant to say, thanks.

Roger.


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