MPIDR Aff0 question

MPIDR Aff0 question

[Andrew Jones]

Hi Marc and Andre,

I completely understand why reset_mpidr() limits Aff0 to 16, thanks
to Andre's nice comment about ICC_SGIxR. Now, here's my question;
it seems that the Cortex-A{53,57,72} manuals want to further limit
Aff0 to 4, going so far as to say bits 7:2 are RES0. I'm looking
at userspace dictating the MPIDR for KVM. QEMU tries to model the
A57 right now, so to be true to the manual, Aff0 should only address
four PEs, but that would generate a higher trap cost for SGI broadcasts
when using KVM. Sigh... what to do?

Additionally I'm looking at adding support to represent more complex
topologies in the guest MPIDR (sockets/cores/threads). I see Linux
currently expects Aff2:socket, Aff1:core, Aff0:thread when threads
are in use, and Aff1:socket, Aff0:core, when they're not. Assuming
there are never more than 4 threads to a core makes the first
expectation fine, but the second one would easily blow the 2 Aff0
bits alloted, and maybe even a 4 Aff0 bit allotment.

So my current thinking is that always using Aff2:socket, Aff1:cluster,
Aff0:core (no threads allowed) would be nice for KVM, and allowing up
to 16 cores to be addressed in Aff0. As it seems there's no standard
for MPIDR, then that could be the KVM guest "standard".

TCG note: I suppose threads could be allowed there, using
Aff2:socket, Aff1:core, Aff0:thread (no more than 4 threads)

Thanks,
drew
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Re: MPIDR Aff0 question

[Marc Zyngier]

Hi Drew,

On 04/02/16 18:38, Andrew Jones wrote:
> 
> Hi Marc and Andre,
> 
> I completely understand why reset_mpidr() limits Aff0 to 16, thanks
> to Andre's nice comment about ICC_SGIxR. Now, here's my question;
> it seems that the Cortex-A{53,57,72} manuals want to further limit
> Aff0 to 4, going so far as to say bits 7:2 are RES0. I'm looking
> at userspace dictating the MPIDR for KVM. QEMU tries to model the
> A57 right now, so to be true to the manual, Aff0 should only address
> four PEs, but that would generate a higher trap cost for SGI broadcasts
> when using KVM. Sigh... what to do?

There are two things to consider:

- The GICv3 architecture is perfectly happy to address 16 CPUs at Aff0.
- ARM cores are designed to be grouped in clusters of at most 4, but
other implementations may have very different layouts.

If you want to model something matches reality, then you have to follow
what Cortex-A cores do, assuming you are exposing Cortex-A cores. But
absolutely nothing forces you to (after all, we're not exposing the
intricacies of L2 caches, which is the actual reason why we have
clusters of 4 cores).

> Additionally I'm looking at adding support to represent more complex
> topologies in the guest MPIDR (sockets/cores/threads). I see Linux
> currently expects Aff2:socket, Aff1:core, Aff0:thread when threads
> are in use, and Aff1:socket, Aff0:core, when they're not. Assuming
> there are never more than 4 threads to a core makes the first
> expectation fine, but the second one would easily blow the 2 Aff0
> bits alloted, and maybe even a 4 Aff0 bit allotment.
> 
> So my current thinking is that always using Aff2:socket, Aff1:cluster,
> Aff0:core (no threads allowed) would be nice for KVM, and allowing up
> to 16 cores to be addressed in Aff0. As it seems there's no standard
> for MPIDR, then that could be the KVM guest "standard".
> 
> TCG note: I suppose threads could be allowed there, using
> Aff2:socket, Aff1:core, Aff0:thread (no more than 4 threads)

I'm not sure why you'd want to map a given topology to a guest (other
than to give the illusion of a particular system). The affinity register
does not define any of this (as you noticed). And what would Aff3 be in
your design? Shelve? Rack? ;-)

What would the benefit of defining a "socket"?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...
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Re: MPIDR Aff0 question

[Andrew Jones]

On Thu, Feb 04, 2016 at 06:51:06PM +0000, Marc Zyngier wrote:
> Hi Drew,
> 
> On 04/02/16 18:38, Andrew Jones wrote:
> > 
> > Hi Marc and Andre,
> > 
> > I completely understand why reset_mpidr() limits Aff0 to 16, thanks
> > to Andre's nice comment about ICC_SGIxR. Now, here's my question;
> > it seems that the Cortex-A{53,57,72} manuals want to further limit
> > Aff0 to 4, going so far as to say bits 7:2 are RES0. I'm looking
> > at userspace dictating the MPIDR for KVM. QEMU tries to model the
> > A57 right now, so to be true to the manual, Aff0 should only address
> > four PEs, but that would generate a higher trap cost for SGI broadcasts
> > when using KVM. Sigh... what to do?
> 
> There are two things to consider:
> 
> - The GICv3 architecture is perfectly happy to address 16 CPUs at Aff0.
> - ARM cores are designed to be grouped in clusters of at most 4, but
> other implementations may have very different layouts.
> 
> If you want to model something matches reality, then you have to follow
> what Cortex-A cores do, assuming you are exposing Cortex-A cores. But
> absolutely nothing forces you to (after all, we're not exposing the
> intricacies of L2 caches, which is the actual reason why we have
> clusters of 4 cores).

Thanks Marc. I'll take the question of whether or not deviation, in
the interest of optimal gicv3 use, is OK to QEMU.

> 
> > Additionally I'm looking at adding support to represent more complex
> > topologies in the guest MPIDR (sockets/cores/threads). I see Linux
> > currently expects Aff2:socket, Aff1:core, Aff0:thread when threads
> > are in use, and Aff1:socket, Aff0:core, when they're not. Assuming
> > there are never more than 4 threads to a core makes the first
> > expectation fine, but the second one would easily blow the 2 Aff0
> > bits alloted, and maybe even a 4 Aff0 bit allotment.
> > 
> > So my current thinking is that always using Aff2:socket, Aff1:cluster,
> > Aff0:core (no threads allowed) would be nice for KVM, and allowing up
> > to 16 cores to be addressed in Aff0. As it seems there's no standard
> > for MPIDR, then that could be the KVM guest "standard".
> > 
> > TCG note: I suppose threads could be allowed there, using
> > Aff2:socket, Aff1:core, Aff0:thread (no more than 4 threads)
> 
> I'm not sure why you'd want to map a given topology to a guest (other
> than to give the illusion of a particular system). The affinity register
> does not define any of this (as you noticed). And what would Aff3 be in
> your design? Shelve? Rack? ;-)

:-) Currently Aff3 would be unused, as there doesn't seem to be a need
for it, and as some processors don't have it, it would only complicate
things to use it sometimes.

> 
> What would the benefit of defining a "socket"?

That's a good lead in for my next question. While I don't believe
there needs to be any relationship between socket and numa node, I
suspect on real machines there is, and quite possibly socket == node.
Shannon is adding numa support to QEMU right now. Without special
configuration there's no gain other than illusion, but with pinning,
etc. the guest numa nodes will map to host nodes, and thus passing
that information on to the guest's kernel is useful. Populating a
socket/node affinity field seems to me like a needed step. But,
question time, is it? Maybe not. Also, the way Linux currently
handles non-thread using MPIDRs (Aff1:socket, Aff0:core) throws a
wrench at the Aff2:socket, Aff1:"cluster", Aff0:core(max 16) plan.
Either the plan or Linux would need to be changed.

Thanks,
drew
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Re: MPIDR Aff0 question

[Marc Zyngier]

On 05/02/16 09:23, Andrew Jones wrote:
> On Thu, Feb 04, 2016 at 06:51:06PM +0000, Marc Zyngier wrote:
>> Hi Drew,
>>
>> On 04/02/16 18:38, Andrew Jones wrote:
>>>
>>> Hi Marc and Andre,
>>>
>>> I completely understand why reset_mpidr() limits Aff0 to 16, thanks
>>> to Andre's nice comment about ICC_SGIxR. Now, here's my question;
>>> it seems that the Cortex-A{53,57,72} manuals want to further limit
>>> Aff0 to 4, going so far as to say bits 7:2 are RES0. I'm looking
>>> at userspace dictating the MPIDR for KVM. QEMU tries to model the
>>> A57 right now, so to be true to the manual, Aff0 should only address
>>> four PEs, but that would generate a higher trap cost for SGI broadcasts
>>> when using KVM. Sigh... what to do?
>>
>> There are two things to consider:
>>
>> - The GICv3 architecture is perfectly happy to address 16 CPUs at Aff0.
>> - ARM cores are designed to be grouped in clusters of at most 4, but
>> other implementations may have very different layouts.
>>
>> If you want to model something matches reality, then you have to follow
>> what Cortex-A cores do, assuming you are exposing Cortex-A cores. But
>> absolutely nothing forces you to (after all, we're not exposing the
>> intricacies of L2 caches, which is the actual reason why we have
>> clusters of 4 cores).
> 
> Thanks Marc. I'll take the question of whether or not deviation, in
> the interest of optimal gicv3 use, is OK to QEMU.
> 
>>
>>> Additionally I'm looking at adding support to represent more complex
>>> topologies in the guest MPIDR (sockets/cores/threads). I see Linux
>>> currently expects Aff2:socket, Aff1:core, Aff0:thread when threads
>>> are in use, and Aff1:socket, Aff0:core, when they're not. Assuming
>>> there are never more than 4 threads to a core makes the first
>>> expectation fine, but the second one would easily blow the 2 Aff0
>>> bits alloted, and maybe even a 4 Aff0 bit allotment.
>>>
>>> So my current thinking is that always using Aff2:socket, Aff1:cluster,
>>> Aff0:core (no threads allowed) would be nice for KVM, and allowing up
>>> to 16 cores to be addressed in Aff0. As it seems there's no standard
>>> for MPIDR, then that could be the KVM guest "standard".
>>>
>>> TCG note: I suppose threads could be allowed there, using
>>> Aff2:socket, Aff1:core, Aff0:thread (no more than 4 threads)
>>
>> I'm not sure why you'd want to map a given topology to a guest (other
>> than to give the illusion of a particular system). The affinity register
>> does not define any of this (as you noticed). And what would Aff3 be in
>> your design? Shelve? Rack? ;-)
> 
> :-) Currently Aff3 would be unused, as there doesn't seem to be a need
> for it, and as some processors don't have it, it would only complicate
> things to use it sometimes.

Careful: on a 64bit CPU, Aff3 is always present.

>>
>> What would the benefit of defining a "socket"?
> 
> That's a good lead in for my next question. While I don't believe
> there needs to be any relationship between socket and numa node, I
> suspect on real machines there is, and quite possibly socket == node.
> Shannon is adding numa support to QEMU right now. Without special
> configuration there's no gain other than illusion, but with pinning,
> etc. the guest numa nodes will map to host nodes, and thus passing
> that information on to the guest's kernel is useful. Populating a
> socket/node affinity field seems to me like a needed step. But,
> question time, is it? Maybe not. Also, the way Linux currently
> handles non-thread using MPIDRs (Aff1:socket, Aff0:core) throws a
> wrench at the Aff2:socket, Aff1:"cluster", Aff0:core(max 16) plan.
> Either the plan or Linux would need to be changed.

What I'm worried of at that stage is that we hardcode a virtual topology
without the knowledge of the physical one. Let's take an example:

I (wish I) have a physical system with 2 sockets, 16 cores per socket, 8
threads per core. I'm about to run a VM with 16 vcpus. If we're going to
start pinning things, then we'll have to express that pinning in the
VM's MPIDRs, and make sure we describe the mapping between the MPIDRs
and the topology in the firmware tables (DT or ACPI).

What I'm trying to say here is that there is you cannot really enforce a
partitioning of MPIDR without considering the underlying HW, and
communicating your expectations to the OS running in the VM.

Do I make any sense?

	M.
-- 
Jazz is not dead. It just smells funny...
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Re: MPIDR Aff0 question

[Mark Rutland]

On Fri, Feb 05, 2016 at 10:23:53AM +0100, Andrew Jones wrote:
> On Thu, Feb 04, 2016 at 06:51:06PM +0000, Marc Zyngier wrote:
 > What would the benefit of defining a "socket"?
> 
> That's a good lead in for my next question. While I don't believe
> there needs to be any relationship between socket and numa node, I
> suspect on real machines there is, and quite possibly socket == node.
> Shannon is adding numa support to QEMU right now. Without special
> configuration there's no gain other than illusion, but with pinning,
> etc. the guest numa nodes will map to host nodes, and thus passing
> that information on to the guest's kernel is useful. Populating a
> socket/node affinity field seems to me like a needed step. But,
> question time, is it? Maybe not. 

I don't think it's necessary.

When using ACPI, NUMA info comes from SRAT+SLIT, and the MPIDR.Aff*
fields do not provide NUMA topology info. I expect the same to be true
with DT using something like numa-distance-map [1].

> Also, the way Linux currently handles non-thread using MPIDRs
> (Aff1:socket, Aff0:core) throws a wrench at the Aff2:socket,
> Aff1:"cluster", Aff0:core(max 16) plan.  Either the plan or Linux
> would need to be changed.

The topology can be explicitly overridden in DT using cpu-map [2]. I
don't know what the story for ACPI is.

Mark.

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-February/404057.html
[2] https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/arm/topology.txt?h=v4.5-rc2&id=36f90b0a2ddd60823fe193a85e60ff1906c2a9b3
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Re: [Qemu-arm] MPIDR Aff0 question

[Andrew Jones]

On Fri, Feb 05, 2016 at 10:37:42AM +0000, Marc Zyngier wrote:
> On 05/02/16 09:23, Andrew Jones wrote:
> > On Thu, Feb 04, 2016 at 06:51:06PM +0000, Marc Zyngier wrote:
> >> Hi Drew,
> >>
> >> On 04/02/16 18:38, Andrew Jones wrote:
> >>>
> >>> Hi Marc and Andre,
> >>>
> >>> I completely understand why reset_mpidr() limits Aff0 to 16, thanks
> >>> to Andre's nice comment about ICC_SGIxR. Now, here's my question;
> >>> it seems that the Cortex-A{53,57,72} manuals want to further limit
> >>> Aff0 to 4, going so far as to say bits 7:2 are RES0. I'm looking
> >>> at userspace dictating the MPIDR for KVM. QEMU tries to model the
> >>> A57 right now, so to be true to the manual, Aff0 should only address
> >>> four PEs, but that would generate a higher trap cost for SGI broadcasts
> >>> when using KVM. Sigh... what to do?
> >>
> >> There are two things to consider:
> >>
> >> - The GICv3 architecture is perfectly happy to address 16 CPUs at Aff0.
> >> - ARM cores are designed to be grouped in clusters of at most 4, but
> >> other implementations may have very different layouts.
> >>
> >> If you want to model something matches reality, then you have to follow
> >> what Cortex-A cores do, assuming you are exposing Cortex-A cores. But
> >> absolutely nothing forces you to (after all, we're not exposing the
> >> intricacies of L2 caches, which is the actual reason why we have
> >> clusters of 4 cores).
> > 
> > Thanks Marc. I'll take the question of whether or not deviation, in
> > the interest of optimal gicv3 use, is OK to QEMU.
> > 
> >>
> >>> Additionally I'm looking at adding support to represent more complex
> >>> topologies in the guest MPIDR (sockets/cores/threads). I see Linux
> >>> currently expects Aff2:socket, Aff1:core, Aff0:thread when threads
> >>> are in use, and Aff1:socket, Aff0:core, when they're not. Assuming
> >>> there are never more than 4 threads to a core makes the first
> >>> expectation fine, but the second one would easily blow the 2 Aff0
> >>> bits alloted, and maybe even a 4 Aff0 bit allotment.
> >>>
> >>> So my current thinking is that always using Aff2:socket, Aff1:cluster,
> >>> Aff0:core (no threads allowed) would be nice for KVM, and allowing up
> >>> to 16 cores to be addressed in Aff0. As it seems there's no standard
> >>> for MPIDR, then that could be the KVM guest "standard".
> >>>
> >>> TCG note: I suppose threads could be allowed there, using
> >>> Aff2:socket, Aff1:core, Aff0:thread (no more than 4 threads)
> >>
> >> I'm not sure why you'd want to map a given topology to a guest (other
> >> than to give the illusion of a particular system). The affinity register
> >> does not define any of this (as you noticed). And what would Aff3 be in
> >> your design? Shelve? Rack? ;-)
> > 
> > :-) Currently Aff3 would be unused, as there doesn't seem to be a need
> > for it, and as some processors don't have it, it would only complicate
> > things to use it sometimes.
> 
> Careful: on a 64bit CPU, Aff3 is always present.

A57 and A72 don't appear to define it though. They have 63:32 as RES0.

> 
> >>
> >> What would the benefit of defining a "socket"?
> > 
> > That's a good lead in for my next question. While I don't believe
> > there needs to be any relationship between socket and numa node, I
> > suspect on real machines there is, and quite possibly socket == node.
> > Shannon is adding numa support to QEMU right now. Without special
> > configuration there's no gain other than illusion, but with pinning,
> > etc. the guest numa nodes will map to host nodes, and thus passing
> > that information on to the guest's kernel is useful. Populating a
> > socket/node affinity field seems to me like a needed step. But,
> > question time, is it? Maybe not. Also, the way Linux currently
> > handles non-thread using MPIDRs (Aff1:socket, Aff0:core) throws a
> > wrench at the Aff2:socket, Aff1:"cluster", Aff0:core(max 16) plan.
> > Either the plan or Linux would need to be changed.
> 
> What I'm worried of at that stage is that we hardcode a virtual topology
> without the knowledge of the physical one. Let's take an example:

Mark's pointer to cpu-map was the piece I was missing. I didn't want to
hardcode anything, but thought we had to at least agree on the meanings
of affinity levels. I see now that the cpu-map node allows us to describe
the meanings.

> 
> I (wish I) have a physical system with 2 sockets, 16 cores per socket, 8
> threads per core. I'm about to run a VM with 16 vcpus. If we're going to
> start pinning things, then we'll have to express that pinning in the
> VM's MPIDRs, and make sure we describe the mapping between the MPIDRs
> and the topology in the firmware tables (DT or ACPI).
> 
> What I'm trying to say here is that there is you cannot really enforce a
> partitioning of MPIDR without considering the underlying HW, and
> communicating your expectations to the OS running in the VM.
> 
> Do I make any sense?

Sure does, but, just be to be sure; so it's not crazy to want to do
this; we just need to 1) pick a topology that makes sense for the
guest/host (that's the user's/libvirt's job), and 2) make sure we
not only assign MPIDR affinities appropriately, but also describe
them with cpu-map (or the ACPI equivalent).

Is that correct?

Thanks,
drew

Re: MPIDR Aff0 question

[Andrew Jones]

On Fri, Feb 05, 2016 at 11:00:33AM +0000, Mark Rutland wrote:
> On Fri, Feb 05, 2016 at 10:23:53AM +0100, Andrew Jones wrote:
> > On Thu, Feb 04, 2016 at 06:51:06PM +0000, Marc Zyngier wrote:
>  > What would the benefit of defining a "socket"?
> > 
> > That's a good lead in for my next question. While I don't believe
> > there needs to be any relationship between socket and numa node, I
> > suspect on real machines there is, and quite possibly socket == node.
> > Shannon is adding numa support to QEMU right now. Without special
> > configuration there's no gain other than illusion, but with pinning,
> > etc. the guest numa nodes will map to host nodes, and thus passing
> > that information on to the guest's kernel is useful. Populating a
> > socket/node affinity field seems to me like a needed step. But,
> > question time, is it? Maybe not. 
> 
> I don't think it's necessary.
> 
> When using ACPI, NUMA info comes from SRAT+SLIT, and the MPIDR.Aff*
> fields do not provide NUMA topology info. I expect the same to be true
> with DT using something like numa-distance-map [1].

Thanks Mark. So it appears my NUMA connection was just muddying the
water. Modeling sockets may or may not have any value to a guest,
but in any case it's a separate issue.

> 
> > Also, the way Linux currently handles non-thread using MPIDRs
> > (Aff1:socket, Aff0:core) throws a wrench at the Aff2:socket,
> > Aff1:"cluster", Aff0:core(max 16) plan.  Either the plan or Linux
> > would need to be changed.
> 
> The topology can be explicitly overridden in DT using cpu-map [2]. I
> don't know what the story for ACPI is.

Thanks for the cpu-map pointer. That was indeed the piece I'd missed
that allows me to make sense of MPIDR affinity level use. I think I'll
still look into modeling sockets/cores/threads with QEMU, by also
adding cpu-map generation. I'll look into what the ACPI equivalent is
as well.

drew

> 
> Mark.
> 
> [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-February/404057.html
> [2] https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/arm/topology.txt?h=v4.5-rc2&id=36f90b0a2ddd60823fe193a85e60ff1906c2a9b3
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Re: MPIDR Aff0 question

[Marc Zyngier]

On 05/02/16 12:03, Andrew Jones wrote:
> On Fri, Feb 05, 2016 at 10:37:42AM +0000, Marc Zyngier wrote:
>> On 05/02/16 09:23, Andrew Jones wrote:
>>> On Thu, Feb 04, 2016 at 06:51:06PM +0000, Marc Zyngier wrote:
>>>> Hi Drew,
>>>>
>>>> On 04/02/16 18:38, Andrew Jones wrote:
>>>>>
>>>>> Hi Marc and Andre,
>>>>>
>>>>> I completely understand why reset_mpidr() limits Aff0 to 16, thanks
>>>>> to Andre's nice comment about ICC_SGIxR. Now, here's my question;
>>>>> it seems that the Cortex-A{53,57,72} manuals want to further limit
>>>>> Aff0 to 4, going so far as to say bits 7:2 are RES0. I'm looking
>>>>> at userspace dictating the MPIDR for KVM. QEMU tries to model the
>>>>> A57 right now, so to be true to the manual, Aff0 should only address
>>>>> four PEs, but that would generate a higher trap cost for SGI broadcasts
>>>>> when using KVM. Sigh... what to do?
>>>>
>>>> There are two things to consider:
>>>>
>>>> - The GICv3 architecture is perfectly happy to address 16 CPUs at Aff0.
>>>> - ARM cores are designed to be grouped in clusters of at most 4, but
>>>> other implementations may have very different layouts.
>>>>
>>>> If you want to model something matches reality, then you have to follow
>>>> what Cortex-A cores do, assuming you are exposing Cortex-A cores. But
>>>> absolutely nothing forces you to (after all, we're not exposing the
>>>> intricacies of L2 caches, which is the actual reason why we have
>>>> clusters of 4 cores).
>>>
>>> Thanks Marc. I'll take the question of whether or not deviation, in
>>> the interest of optimal gicv3 use, is OK to QEMU.
>>>
>>>>
>>>>> Additionally I'm looking at adding support to represent more complex
>>>>> topologies in the guest MPIDR (sockets/cores/threads). I see Linux
>>>>> currently expects Aff2:socket, Aff1:core, Aff0:thread when threads
>>>>> are in use, and Aff1:socket, Aff0:core, when they're not. Assuming
>>>>> there are never more than 4 threads to a core makes the first
>>>>> expectation fine, but the second one would easily blow the 2 Aff0
>>>>> bits alloted, and maybe even a 4 Aff0 bit allotment.
>>>>>
>>>>> So my current thinking is that always using Aff2:socket, Aff1:cluster,
>>>>> Aff0:core (no threads allowed) would be nice for KVM, and allowing up
>>>>> to 16 cores to be addressed in Aff0. As it seems there's no standard
>>>>> for MPIDR, then that could be the KVM guest "standard".
>>>>>
>>>>> TCG note: I suppose threads could be allowed there, using
>>>>> Aff2:socket, Aff1:core, Aff0:thread (no more than 4 threads)
>>>>
>>>> I'm not sure why you'd want to map a given topology to a guest (other
>>>> than to give the illusion of a particular system). The affinity register
>>>> does not define any of this (as you noticed). And what would Aff3 be in
>>>> your design? Shelve? Rack? ;-)
>>>
>>> :-) Currently Aff3 would be unused, as there doesn't seem to be a need
>>> for it, and as some processors don't have it, it would only complicate
>>> things to use it sometimes.
>>
>> Careful: on a 64bit CPU, Aff3 is always present.
> 
> A57 and A72 don't appear to define it though. They have 63:32 as RES0.

That's because they do support AArch32, which only has Aff2-0. On a pure
64bit CPU, Aff3 would definitely have an Aff3 (though it is most likely
to be 0 for a while).

>>
>>>>
>>>> What would the benefit of defining a "socket"?
>>>
>>> That's a good lead in for my next question. While I don't believe
>>> there needs to be any relationship between socket and numa node, I
>>> suspect on real machines there is, and quite possibly socket == node.
>>> Shannon is adding numa support to QEMU right now. Without special
>>> configuration there's no gain other than illusion, but with pinning,
>>> etc. the guest numa nodes will map to host nodes, and thus passing
>>> that information on to the guest's kernel is useful. Populating a
>>> socket/node affinity field seems to me like a needed step. But,
>>> question time, is it? Maybe not. Also, the way Linux currently
>>> handles non-thread using MPIDRs (Aff1:socket, Aff0:core) throws a
>>> wrench at the Aff2:socket, Aff1:"cluster", Aff0:core(max 16) plan.
>>> Either the plan or Linux would need to be changed.
>>
>> What I'm worried of at that stage is that we hardcode a virtual topology
>> without the knowledge of the physical one. Let's take an example:
> 
> Mark's pointer to cpu-map was the piece I was missing. I didn't want to
> hardcode anything, but thought we had to at least agree on the meanings
> of affinity levels. I see now that the cpu-map node allows us to describe
> the meanings.
> 
>>
>> I (wish I) have a physical system with 2 sockets, 16 cores per socket, 8
>> threads per core. I'm about to run a VM with 16 vcpus. If we're going to
>> start pinning things, then we'll have to express that pinning in the
>> VM's MPIDRs, and make sure we describe the mapping between the MPIDRs
>> and the topology in the firmware tables (DT or ACPI).
>>
>> What I'm trying to say here is that there is you cannot really enforce a
>> partitioning of MPIDR without considering the underlying HW, and
>> communicating your expectations to the OS running in the VM.
>>
>> Do I make any sense?
> 
> Sure does, but, just be to be sure; so it's not crazy to want to do
> this; we just need to 1) pick a topology that makes sense for the
> guest/host (that's the user's/libvirt's job), and 2) make sure we
> not only assign MPIDR affinities appropriately, but also describe
> them with cpu-map (or the ACPI equivalent).
> 
> Is that correct?

I believe so. That way, you can describe to the guest OS what are the
constraints you have put on the VM from the host, and it itself can make
an informed decision on task placement and such.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...
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