linux-kernel - Re: [RFC v2 2/4] iommu/arm-smmu-v3: Add tlbi_on

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Message-ID: <cc18b752-97fd-f95f-fdef-478adc06e505@redhat.com>
Date:   Wed, 23 Aug 2017 14:36:53 +0200
From:   Auger Eric <eric.auger@...hat.com>
To:     Will Deacon <will.deacon@....com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     eric.auger.pro@...il.com, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, robin.murphy@....com,
        Jean-Philippe.Brucker@....com, christoffer.dall@...aro.org,
        Marc.Zyngier@....com, alex.williamson@...hat.com,
        peterx@...hat.com, tn@...ihalf.com, bharat.bhushan@....com
Subject: Re: [RFC v2 2/4] iommu/arm-smmu-v3: Add tlbi_on_map option

Hi Will,

On 23/08/2017 12:25, Will Deacon wrote:
> On Tue, Aug 22, 2017 at 10:09:15PM +0300, Michael S. Tsirkin wrote:
>> On Fri, Aug 18, 2017 at 05:49:42AM +0300, Michael S. Tsirkin wrote:
>>> On Thu, Aug 17, 2017 at 05:34:25PM +0100, Will Deacon wrote:
>>>> On Fri, Aug 11, 2017 at 03:45:28PM +0200, Eric Auger wrote:
>>>>> When running a virtual SMMU on a guest we sometimes need to trap
>>>>> all changes to the translation structures. This is especially useful
>>>>> to integrate with VFIO. This patch adds a new option that forces
>>>>> the IO_PGTABLE_QUIRK_TLBI_ON_MAP to be applied on LPAE page tables.
>>>>>
>>>>> TLBI commands then can be trapped.
>>>>>
>>>>> Signed-off-by: Eric Auger <eric.auger@...hat.com>
>>>>>
>>>>> ---
>>>>> v1 -> v2:
>>>>> - rebase on v4.13-rc2
>>>>> ---
>>>>>  Documentation/devicetree/bindings/iommu/arm,smmu-v3.txt | 4 ++++
>>>>>  drivers/iommu/arm-smmu-v3.c                             | 5 +++++
>>>>>  2 files changed, 9 insertions(+)
>>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/iommu/arm,smmu-v3.txt b/Documentation/devicetree/bindings/iommu/arm,smmu-v3.txt
>>>>> index c9abbf3..ebb85e9 100644
>>>>> --- a/Documentation/devicetree/bindings/iommu/arm,smmu-v3.txt
>>>>> +++ b/Documentation/devicetree/bindings/iommu/arm,smmu-v3.txt
>>>>> @@ -52,6 +52,10 @@ the PCIe specification.
>>>>>                          devicetree/bindings/interrupt-controller/msi.txt
>>>>>                        for a description of the msi-parent property.
>>>>>  
>>>>> +- tlbi-on-map       : invalidate caches whenever there is an update of
>>>>> +                      any remapping structure (updates to not-present or
>>>>> +                      present entries).
>>>>> +
>>>>
>>>> My position on this hasn't changed, so NAK for this patch. If you want to
>>>> emulate something outside of the SMMUv3 architecture, please do so, but
>>>> don't pretend that it's an SMMUv3.
>>>>
>>>> Will
>>>
>>> What if the emulated device does not list arm,smmu-v3, listing
>>> qemu,ssmu-v3 as compatible? Would that address the concern?
>>
>> Will, can you comment on this please? Are you open to reusing the code
>> in drivers/iommu/arm-smmu-v3.c to support a paravirtual device that does
>> not claim to be compatible with smmuv3 but does try to behave very close to
>> it except it can cache non-present structures? Or would you rather
>> the code to support this is forked to qemu-smmu-v3.c?
> 
> I still don't understand why this is preferable to a PV IOMMU
> implementation. Not only is this proposing to issue TLB maintenance on
> map, but the maintenance command itself is entirely made up. Why not just
> have a map command? Anyway, I'm reluctant to add this hack to the driver until:
> 
>   1. There is a compelling reason to pursue this approach instead of a
>      PV approach (including performance measurements).
> 
>   2. There is a specification for the QEMU fork of the ARM SMMUv3
>      architecture, including the semantics of the new command being proposed
>      and what exactly the TLB maintenance requirements are on map (for
>      example, what if I change an STE or a CD -- are they cached too?).
I am not sure I catch this last point. At the moment whenever the smmuv3
driver issues data structure invalidation commands (CMD_CFGI_*), those
are trapped and I replay the mappings on host side. I have not changed
anything on that side.

I introduced a new map implementation defined command because the per
page CMD_TLBI_NH_VA IOVA invalidation command was not efficient/usable
with use cases such as DPDK on guest. I understood the spec provisions
for such implementation defined commands.
> 
>   3. The ACPI IORT spec is updated to recognise this implementation
> 
>   4. There is an implementation that can use the guest page tables directly,
>      because that may well make all of this moot.
Most probably I will come back to you with questions on stage 1 + stage2
enablement and "4.8 Virtualisation" chapter of smmuv3 spec. Besides I
also need to get access to some HW with smmuv3 ;-)

Thanks

Eric
> 
> Forking the driver doesn't sound very sensible to me.
> 
> Will
>