[Qemu-devel] spapr guests warn "System page size 0x2000 is not enabled in page_size_mask (0x11000). Performance may be slow" on SPARC hosts

[Qemu-devel] spapr guests warn "System page size 0x2000 is not enabled in page_size_mask (0x11000). Performance may be slow" on SPARC hosts

[Peter Maydell]

I've just noticed that on a SPARC host, some of the PPC guests
warn during make check:

  /ppc64/prom-env/pseries:
qemu-system-ppc64: System page size 0x2000 is not enabled in
page_size_mask (0x11000). Performance may be slow

Is this really a performance problem on a TCG guest?
It makes the 'make check' output a bit noisy, but not
unbearably so.

thanks
-- PMM

Re: [Qemu-devel] spapr guests warn "System page size 0x2000 is not enabled in page_size_mask (0x11000). Performance may be slow" on SPARC hosts

[Alexander Graf]

On 06/15/2017 03:33 PM, Peter Maydell wrote:
> I've just noticed that on a SPARC host, some of the PPC guests
> warn during make check:
>
>    /ppc64/prom-env/pseries:
> qemu-system-ppc64: System page size 0x2000 is not enabled in
> page_size_mask (0x11000). Performance may be slow
>
> Is this really a performance problem on a TCG guest?
> It makes the 'make check' output a bit noisy, but not
> unbearably so.

 From what I can tell this really only affects DDW which should be page 
size agnostic when only using emulated devices. Maybe Alexey thought of 
using VFIO-pci devices in a TCG guest?


Alex

Re: [Qemu-devel] spapr guests warn "System page size 0x2000 is not enabled in page_size_mask (0x11000). Performance may be slow" on SPARC hosts

[Thomas Huth]

On 19.06.2017 11:39, Alexander Graf wrote:
> On 06/15/2017 03:33 PM, Peter Maydell wrote:
>> I've just noticed that on a SPARC host, some of the PPC guests
>> warn during make check:
>>
>>    /ppc64/prom-env/pseries:
>> qemu-system-ppc64: System page size 0x2000 is not enabled in
>> page_size_mask (0x11000). Performance may be slow
>>
>> Is this really a performance problem on a TCG guest?
>> It makes the 'make check' output a bit noisy, but not
>> unbearably so.
> 
> From what I can tell this really only affects DDW which should be page
> size agnostic when only using emulated devices. Maybe Alexey thought of
> using VFIO-pci devices in a TCG guest?

Maybe we could at least silence the warning by checking qtest_enabled()?

 Thomas

Re: [Qemu-devel] spapr guests warn "System page size 0x2000 is not enabled in page_size_mask (0x11000). Performance may be slow" on SPARC hosts

[David Gibson]

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On Mon, Jun 19, 2017 at 12:32:48PM +0200, Thomas Huth wrote:
> On 19.06.2017 11:39, Alexander Graf wrote:
> > On 06/15/2017 03:33 PM, Peter Maydell wrote:
> >> I've just noticed that on a SPARC host, some of the PPC guests
> >> warn during make check:
> >>
> >>    /ppc64/prom-env/pseries:
> >> qemu-system-ppc64: System page size 0x2000 is not enabled in
> >> page_size_mask (0x11000). Performance may be slow
> >>
> >> Is this really a performance problem on a TCG guest?
> >> It makes the 'make check' output a bit noisy, but not
> >> unbearably so.
> > 
> > From what I can tell this really only affects DDW which should be page
> > size agnostic when only using emulated devices. Maybe Alexey thought of
> > using VFIO-pci devices in a TCG guest?
> 
> Maybe we could at least silence the warning by checking
> qtest_enabled()?


So,

1) Yes, we should at least silence the warning.

2) Certainly the mentioned slowdown shouldn't be significant for TCG
guests.

3) I _think_ the restriction which causes the slowdown doesn't even
exist for TCG, but I haven't 100% convinced myself of this.  I've been
sick lately, so my brain isn't working all that well.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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Re: [Qemu-devel] spapr guests warn "System page size 0x2000 is not enabled in page_size_mask (0x11000). Performance may be slow" on SPARC hosts

[Alexey Kardashevskiy]

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On 20/06/17 11:02, David Gibson wrote:
> On Mon, Jun 19, 2017 at 12:32:48PM +0200, Thomas Huth wrote:
>> On 19.06.2017 11:39, Alexander Graf wrote:
>>> On 06/15/2017 03:33 PM, Peter Maydell wrote:
>>>> I've just noticed that on a SPARC host, some of the PPC guests
>>>> warn during make check:
>>>>
>>>>    /ppc64/prom-env/pseries:
>>>> qemu-system-ppc64: System page size 0x2000 is not enabled in
>>>> page_size_mask (0x11000). Performance may be slow
>>>>
>>>> Is this really a performance problem on a TCG guest?
>>>> It makes the 'make check' output a bit noisy, but not
>>>> unbearably so.
>>>
>>> From what I can tell this really only affects DDW which should be page
>>> size agnostic when only using emulated devices. Maybe Alexey thought of
>>> using VFIO-pci devices in a TCG guest?
>>
>> Maybe we could at least silence the warning by checking
>> qtest_enabled()?
> 
> 
> So,
> 
> 1) Yes, we should at least silence the warning.

Agree, wrapping it into kvm_enabled() should do it.


> 
> 2) Certainly the mentioned slowdown shouldn't be significant for TCG
> guests.

It is not significant at all. While in-kernel TCE handling improves
throughput for VFIO devices quite a lot (200mb/s -> 1000mb/s), TCG could
not do any better than 20mb/s anyway.


> 3) I _think_ the restriction which causes the slowdown doesn't even
> exist for TCG, but I haven't 100% convinced myself of this.  I've been
> sick lately, so my brain isn't working all that well.

Mismatch between system page size and IOMMU page size means H_PUT_TCE is
going to be called more often than needed; if VFIO is involved, then more
ioctl(vfio_container_fd, DMAP_MAP) calls. But - we cannot do 8K IOMMU pages
at all, SPARC does not support VFIO (or does it?), and TCG is slow anyway.



-- 
Alexey


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